CRISPR/Cas and Argonaute-Based Biosensors for Pathogen Detection

Highly sensitive electrochemical Osteoprotegerin (OPG) immunosensor for assessing fracture healing and evaluating drug efficacy

Tibial fractures are common long bone injuries requiring effective monitoring for optimal healing. Osteoprotegerin (OPG), as a key marker of bone formation, is closely related to the degree of fracture healing. However, existing detection methods have certain limitations in sensitivity and specificity. This study successfully crafted an exceptionally sensitive electrochemical immunosensor based on COOH-CNFs/Ti3C2Tx MXene/PANI-AgNPs nanocomposite material for the quantitative analysis of OPG in serum, providing a methodological basis for auxiliary diagnosis of fracture healing degree and evaluation of drug efficacy. A one-pot hydrothermal method was employed to synthesize and modify the nanocomposite material on gold electrode surfaces, which exhibit high electrochemical activity, low charge transfer resistance, and a large electroactive surface area, thereby enhancing the immunosensor's conductivity and stability, with a wide linear range (10-17 to 10-12 g/mL) and a low detection limit (1.94 × 10-18 g/mL). Methodological validation further confirmed the immunosensor's excellent performance in specificity, reproducibility, and stability. Moreover, the successful application of this immunosensor in detecting OPG in serum samples from actual tibial fracture patients before and after medication demonstrates significant potential for clinical application in assisting the assessment of fracture healing and evaluating the efficacy of orthopedic drugs.

Argonaute protein powered biosensing for pathogenic biosafety

The food safety and medical health issues caused by pathogen are particularly prominent. The development of biosensing technologies is urgent to ensure pathogenic biosafety. Argonaute system, as a promising and cutting-edged next-generation nucleic acid test technology, has the potential to address the challenges faced by CRISPR/Cas system. In this review, we focused on the current state-of-art Argonaute-powered biosensing for pathogenic biosafety. First, we introduced current methods for nucleic acid testing and programmable nucleases, followed by the working principle of Argonaute system (PfAgo, TtAgo, CbAgo, etc). Then Argonaute-medicated nucleic acid biosensing was highlighted through amplification and amplification-free manners. In addition, we summarized the application of Argonaute tools in detecting bacteria, virus, mycoplasma, etc. Finally, we pointed out the challenges and perspectives. Current pathogen methods demonstrate low sensitivity and specificity, as well as lack capabilities for multiple and point-of-care testing. Recent studies have shown that Argonaute-powered biosensing is an innovative and rapidly growing technology that could significantly enhance detection capabilities for pathogen-related issues, addressing the limitations of current methods. The application of Argonaute-powered biosensing is both promising and desirable due to the potential to offer "customized" and streamlined detection in the field of pathogenic biosafety monitoring.

CRISPR/Cas and Argonaute-based biosensors for nucleic acid detection

Nowadays, nucleic acid detection technology has been applied to disease diagnosis, prevention, food safety, environmental testing and many other aspects. However, traditional methods still have shortcomings. Therefore, there is an urgent need for a simple, rapid, sensitive, and specific new method to supersede traditional nucleic acid detection technology. CRISPR/Cas(Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated) system and Argonaute (Ago) system play an important role in microbial immune defense. Their targeting specificity, programmability and special trans-cleavage activity make it possible to develop some new platforms for nucleic acid detection in combination with a variety of biosensors. We introduce the origins of these two systems and the biosensors developed based on CRISPR/Cas system and Ago system, respectively, especially the prospects for the future development of Cascade Amplification biosensors. This review is expected to provide useful guidance for researchers in related fields and provide inspiration for the development of Cascade Amplification biosensors in the future.

Programmable AIESTA: All-in-One Isothermal Enzymatic Signal Transduction Amplifier for Portable Profiling

The Argonaute (Ago) protein exhibits high specificity in nucleic acid recognition and cleavage, making it highly promising for biosensing applications. Its potential is further enhanced by its independence from protospacer adjacent motif (PAM) requirements and the cost-effectiveness of using short DNA guides. Both Ago and CRISPR/Cas systems face challenges in signal amplification, which limit their ability to detect targets at ultralow concentrations. To overcome this limitation, a thermostable quadratic amplification system (T-QAS) was constructed by integrating a thermostable nicking-enzyme-mediated amplification (NEMA) strategy with TtAgo. The system leverages the high stability of T-QAS at elevated temperatures to enhance guide-target interactions and decrease false positives caused by nonspecific amplification. Additionally, nanozyme is integrated with T-QAS to construct the AIESTA platform (all-in-one isothermal enzymatic signal transduction amplifier), which is a single-tube visual sensing platform. Within the AIESTA system, T-QAS improves specificity through high operational temperatures and offers programmable functions, enabling the sensitive detection of miRNA and foodborne toxins. The combination of T-QAS and nanozyme makes AIESTA a candidate of point-of-care testing (POCT) field, showcasing the potential for biosensing in resource-limited and complex environments.

A DNA sensor based on CbAgo effector protein and on a dual electrochemical signal amplification strategy for B19 parvovirus detection

Human parvovirus B19 is a prevalent childhood infectious virus that poses a great challenge to public health, so the detection of B19V is of great importance. In this study, a DNA sensor based on CbAgo, a Cas effector, and a dual electrochemical signal amplification strategy was developed by using a novel nanocomposite MnO2/CMK-3/g-C3N4/AgNPs for initial signal amplification, with CMK being an ordered mesoporous carbon nanomaterial. Single-walled carbon nanotubes (SWCNTs) were used as electrocatalytic probes for secondary signal amplification to detect B19 DNA. The detection process begins with polymerase chain reaction (PCR) amplification using the B19V infectious clone plasmid (pB19-M20) as a template and NS1-F/R as primers, followed by specific cleavage of B19 DNA based on the programmable cutting sites of CbAgo effector protein. This study enriches the application of Argonaute proteins in sensing and introduces a novel method to detect B19V. Under optimized conditions, the biosensor can detect B19 DNA in the range of 10-15-10-10 M, with a detection limit (LOD) of 0.2 fM. The results indicate that the developed DNA sensor holds promise for reliable and sensitive detection of B19 DNA in human serum.

TtrAgo-mediated nucleic acid detection system and portable device for rapid detection of sexually transmitted diseases

The development of rapid and multiplexed point-of-care (POC) diagnostic tools is vital for the prevention and control of sexually transmitted diseases (STIs). Here, we developed a POC-comprehensive Thermococcus thioreducensArgonaute (TtrAgo)-mediated nucleic acid detection system (POC-CANDY) and palm-sized portable detection device "Owl-1" for the simultaneous detection of Ureaplasma urealyticum, Chlamydia trachomatis, Neisseria gonorrhoeae, human papillomavirus types 16/18 and antibiotic resistance molecular markers [tetM, and gyrA mutation (S91F)]. Using recombinase polymerase amplification (RPA), the optimized POC-CANDY could finish the whole detection procedure within 55 min and achieve a limit of detection of 10 copies/μL. When validated by clinical STI samples, POC-CANDY showed 100% consistency with quantitative PCR. Additionally, compared with the PfAgo-based system, POC-CANDY significantly improved the sensitivity of distinguishing single nucleotide variations. The results demonstrated that POC-CANDY can be easily applied locally or on site. This study also promotes the utility of the TtrAgo-mediated technique in clinical diagnosis.

A CRISPR-Cas and Argonaute-Driven Two-Factor Authentication Strategy for Information Security

The escalating growth in computing power and the advent of quantum computing present a critical threat to the security of modern cryptography. Two-factor authentication strategies can effectively resist brute-force attacks to improve the security of access control. Herein, we proposed a two-factor and two-authentication entity strategy based on the trans-cleavage activity of CRISPR-Cas and the "dual-step" sequence-specific cleavage of Pyrococcus furiosus Argonaute. In this strategy, the output of authentication entity 1 acted as a component to operate authentication entity 2, thus enabling a role-based molecular model that implemented access control for the three roles. To further enhance information security, we designed knowledge suppression factors to constitute the command library and possession suppression factors to resist brute-force attacks. This study will promote the development of advanced molecular access control and its applications in biomedical diagnostics and data security.

Micropore Resistance Counting Platform for Multiplexed and Ultrasensitive Detection of Mycotoxins and Biomarkers

Development of a multiplexed and sensitive biosensing platform is a priority for public health security. We report a micropore resistance counting platform based on polystyrene microsphere size-based encoding and Clostridium butyricum Argonaute (CbAgo) decoding for multiplexed and ultrasensitive detection. Initially, we constructed a target DNA-modified polystyrene microsphere coding system based on micropore resistance counting. Subsequently, the precise recognition and cleavage capabilities of the guide DNA-activated CbAgo protein enable the decoding of the encoded microsphere system. Changes in the concentration of polystyrene microspheres are presented as a signal readout. The platform demonstrated excellent performance in multiplexed detection of three mycotoxins (with a sensitivity range over 4 orders of magnitude reaching the pg/mL level) and two inflammatory markers at pg/mL. Combining precise enzyme cleavage by CbAgo with micropore resistance counting, the developed platform is a multiplexed and highly sensitive detection tool with wide-ranging potential in applications such as clinical diagnosis, food safety inspection, and environmental monitoring.

CRISPR/Cas12a dual-mode biosensor for Staphylococcus aureus detection via enzyme-free isothermal amplification

Accurate and reliable detection of Staphylococcus aureus (S. aureus) is essential for preventing infections, particularly in healthcare and food safety contexts. This work presents a novel dual-mode biosensor that integrates the CRISPR/Cas12a system with an enzyme-free isothermal amplification method for detecting S. aureus. Hybridization chain reaction (HCR) and catalytic hairpin assembly (CHA) amplify the aptamer-triggered response, significantly enhancing sensitivity. CRISPR/Cas12a's nuclease activity is utilized in two modes: cis cleavage generates a fluorescence signal, while trans cleavage produces an electrochemical signal, enabling dual-mode detection. The biosensor demonstrates outstanding performance, with a limit of detection (LOD) as low as 5.7 CFU mL-1 in electrochemical mode and 133.7 CFU mL-1 in fluorescence mode, showcasing excellent accuracy, stability, and sensitivity. It has been successfully applied to detecting actual samples, confirming its practical applicability. This innovative approach offers a powerful tool for the swift and precise identification of S. aureus and paves the way for developing next-generation dual-mode biosensors for various analytes. Future research will aim to simplify the detection process further, making it more accessible for use in resource-limited settings.

Using Recombinase-Aid Amplification Combined with Pyrococcus furiosus Argonaute for Rapid Sex Identification in Flamingo (Phoenicopteridae)

Flamingos (Phoenicopteridae) are among the oldest birds worldwide and are loved by people for their bright red feathers. In addition, flamingos are sexually monomorphic birds, and distinguishing between males and females is difficult. The polymerase chain reaction (PCR) is widely used for sex identification. However, the PCR method requires a precise thermal cycler in the laboratory and is time-consuming. Therefore, developing a rapid, sensitive, and accurate method to identify the sex of flamingos is crucial. In this study, we established a sex identification system using a recombinase-aided amplification-Pyrococcus furiosus Argonaute (RAA-PfAgo) technique for greater flamingo (Phoenicopterus roseus). The greater flamingo-RAA-PfAgo system can identify unknown-sex greater flamingos in less than 1 h and can be visualized using a fluorescent detector or blue light. The results showed that optimal RAA-PfAgo conditions could detect 0.6 ng of genomic DNA and effectively differentiate between males and females. Random sample evaluations revealed that the system had a 100% coincidence rate compared with conventional PCR. In conclusion, this study provides a sensitive, specific, and accurate reference method for greater flamingo sexing.

MIRA/PfAgo-Mediated Biosensor for Multiplex Human Enteroviruses Virus Typing Detection on HFMD

Hand, foot, and mouth disease (HFMD), caused by enteroviruses, mostly including EV71, CVA6, CVA10, and CVA16, is an acute infectious disease commonly found in children. Due to no approved antiviral therapies and available vaccines, except for EV71, developing accurate diagnostic methods of HFMD is essential for controlling its spread and mitigating its impact on public health. Here, we create a MIRA-HEV-PAND multiple nucleic acid typing method that utilizes PfAgo to identify enterovirus type A pathogens (EV71, CVA6, CVA10, and CVA16) and universal type EVU. The MDC (minimum detection concentration) level of MIRA-HEV-PAND is within the range of 1.66 aM (1.0 copy/μL), which was matched to that of qPCR assays and even more sensitive up to 10%. Importantly, the MIRA-HEV-PAND method exhibits higher sensitivity and less time-consuming efficiency compared to the approach that combines PCR amplification instead of MIRA amplification. Meanwhile, though the quintuple and single-tube multiple MIRA-HEV-PAND detection system can be used for one viral target or multiple viral target detection, the single-tube detection system detects more efficiently and rapidly than the quintuple-tube multiple detection system. Moreover, the diagnostic results obtained by evaluating clinical samples using MIRA-HEV-PAND show a complete consistency of 100% with qPCR assays. The MIRA-HEV-PAND method can screen a wider range of target regions using low-cost guide DNA without being limited to PAM sequences, compared to the MARPLES based on the CRISPR-Cas12a. The utilization of this correlation can be beneficial for the application of molecular testing for clinical diagnoses and the study of human enteroviruses A infection and virus typing on an epidemiological scale.

Argonaute-Based Nucleic Acid Detection Technology: Advantages, Current Status, Challenges, and Perspectives

Rapid and accurate detection is a prerequisite for precise clinical diagnostics, ensuring food safety, and facilitating biotechnological applications. The Argonaute system, as a cutting-edge technique, has been successfully repurposed in biosensing beyond the CRISPR/Cas system (clustered regularly interspaced short palindromic repeats and CRISPR-associated proteins), which has been extensively researched, but recognition of PAM sequences remains restricted. Argonaute, as a programmable and target-activated nuclease, is repurposed for fabricating novel detection methods due to its unparalleled biological features. In this comprehensive review, we initially elaborate on the current methods for nucleic acid testing and programmable nucleases, followed by delving into the structure and nuclease activity of the Argonaute system. The advantages of Argonaute compared with the CRISPR/Cas system in nucleic acid detection are highlighted and discussed. Furthermore, we summarize the applications of Argonaute-based nucleic acid detection and provide an in-depth analysis of future perspectives and challenges. Recent research has demonstrated that Argonaute-based biosensing is an innovative and rapidly advancing technology that can overcome the limitations of existing methods and potentially replace them. In summary, the implementation of Argonaute and its integration with other technologies hold promise in developing customized and intelligent detection methods for nucleic acid testing across various aspects.

Programmable and ultra-efficient Argonaute protein-mediated nucleic acid tests: A review

With the attributes of high sensitivity, single-base resolution, multiplex detection capability, and programmability upon nucleic acid recognition, Argonaute (Ago)-based biosensing assays are increasingly recognized as one of the most promising tools for precise identification and quantification of target analytes. Employed as highly specific sequence recognition elements of these robust diagnostic methods, Agos are revolutionizing how nucleic acid targets are detected. A systematic and comprehensive summary of this emerging and rapid-advancing technology is necessary to give play to the potential of Ago-based biosensing assays. The structure and function of Agos were briefly overviewed at the beginning of the work, followed by a review of the recent advancements in employing Agos sensing for detecting various targets with a comprehensive analysis such as viruses, tumor biomarkers, pathogens, mycoplasma, and parasite. The significance and benefits of these platforms were then deliberated. In addition, the authors shared subjective viewpoints on the existing challenges and offered relevant guidance for the future progress of Agos assays. Finally, the future research outlook regarding Ago-based sensing in this field was also outlined. As such, this review is expected to offer valuable information and fresh perspectives for a broader group of researchers.

CRISPR/Cas14 and G-Quadruplex DNAzyme-Driven Biosensor for Paper-Based Colorimetric Detection of African Swine Fever Virus

The highly contagious nature and 100% fatality rate contribute to the ongoing and expanding impact of the African swine fever virus (ASFV), causing significant economic losses worldwide. Herein, we developed a cascaded colorimetric detection using the combination of a CRISPR/Cas14a system, G-quadruplex DNAzyme, and microfluidic paper-based analytical device. This CRISPR/Cas14a-G4 biosensor could detect ASFV as low as 5 copies/μL and differentiate the wild-type and mutated ASFV DNA with 2-nt difference. Moreover, this approach was employed to detect ASFV in porcine plasma. A broad linear detection range was observed, and the limit of detection in spiked porcine plasma was calculated to be as low as 42-85 copies/μL. Our results indicate that the developed paper platform exhibits the advantages of high sensitivity, excellent specificity, and low cost, making it promising for clinical applications in the field of DNA disease detection and suitable for popularization in low-resourced areas.

A one-pot CRISPR-RCA strategy for ultrasensitive and specific detection of circRNA

Accurate and precise detection of circular RNA (circRNA) is imperative for its clinical use. However, the inherent challenges in circRNA detection, arising from its low abundance and potential interference from linear isomers, necessitate innovative solutions. In this study, we introduce, for the first time, the application of the CRISPR/Cas12a system to establish a one-pot, rapid (30 minutes to 2 hours), specific and ultrasensitive circRNA detection strategy, termed RETA-CRISPR (reverse transcription-rolling circle amplification (RT-RCA) with the CRISPR/Cas12a). This method comprises two steps: (1) the RT-RCA process of circRNA amplification, generating repeat units containing the back-splicing junction (BSJ) sequences; and (2) leveraging the protospacer adjacent motif (PAM)-independent Cas12a/crRNA complex to precisely recognize target sequences with BSJ, thereby initiating the collateral cleavage activity of Cas12a to generate a robust fluorescence signal. Remarkably, this approach exhibits the capability to detect circRNAs at a concentration as low as 300 aM. The sensor has been successfully employed for accurate detection of a potential hepatocellular carcinoma biomarker hsa_circ_0001445 (circRNA1445) in various cell lines. In conclusion, RETA-CRISPR seamlessly integrates the advantages of exponential amplification reaction and the robust collateral cleavage activity of Cas12a, positioning it as a compelling tool for practical CRISPR-based diagnostics.

On-Site and Visual Detection of the H5 Subtype Avian Influenza Virus Based on RT-RPA and CRISPR/Cas12a

Avian influenza viruses (AIVs) of the H5 subtype rank among the most serious pathogens, leading to significant economic losses in the global poultry industry and posing risks to human health. Therefore, rapid and accurate virus detection is crucial for the prevention and control of H5 AIVs. In this study, we established a novel detection method for H5 viruses by utilizing the precision of CRISPR/Cas12a and the efficiency of RT-RPA technologies. This assay facilitates the direct visualization of detection results through blue light and lateral flow strips, accurately identifying H5 viruses with high specificity and without cross-reactivity against other AIV subtypes, NDV, IBV, and IBDV. With detection thresholds of 1.9 copies/μL (blue light) and 1.9 × 103 copies/μL (lateral flow strips), our method not only competes with but also slightly surpasses RT-qPCR, demonstrating an 80.70% positive detection rate across 81 clinical samples. The RT-RPA/CRISPR-based detection method is characterized by high sensitivity, specificity, and independence from specialized equipment. The immediate field applicability of the RT-RPA/CRISPR approach underscores its importance as an effective tool for the early detection and management of outbreaks caused by the H5 subtype of AIVs.

CRISPR/Cas12a powered air-displacement enhanced evanescent wave fluorescence fiber-embedded microfluidic biochip for nucleic acid amplification-free detection of Escherichia coli O157:H7

Simple yet ultrasensitive and contamination-free quantification of environmental pathogenic bacteria is in high demand. In this study, we present a portable clustered regularly interspaced short palindromic repeats-associated protein 12a (CRISPR/Cas12a) powered Air-displacement enhanced Evanescent wave fluorescence Fiber-embedded microfluidic Biochip (AEFB) for the high-frequency and nucleic acid amplification-free ultrasensitive detection of Escherichia coli O157:H7. The performance of AEFB was dramatically enhanced upon employing a simple air-solution displacement process. Theoretical assays demonstrated that air-solution displacement significantly enhances evanescent wave field intensity on the fiber biosensor surface and increases the V-number in tapered fiber biosensors. Consequently, light-matter interaction is strengthened, and fluorescence coupling and collection efficiency are improved, considerably enhancing sensitivity. By integrating the CRISPR biosensing mechanism, AEFB facilitated rapid, accurate, nucleic acid amplification-free detection of E.coli O157:H7 with polymerase chain reaction (PCR)-level sensitivity (176 cfu/mL). To validate its practicality, AEFB was used to detect E.coli O157:H7 in surface water and wastewater. Comparison with RT-PCR showed a strong linear relationship (R2 = 0.9871), indicating the excellent accuracy and reliability of this technology in real applications. AEFB is highly versatile and can be easily extended to detect other pathogenic bacteria, which will significantly promote the high-frequency assessment and early-warning of bacterial contamination in aquatic environments.

Exploring T7 RNA polymerase-assisted CRISPR/Cas13a amplification for the detection of BNP via electrochemiluminescence sensing platform

Brain natriuretic peptide (BNP) is considered to be an important biomarker of heart failure (HF) attracting attention. However, its low concentration and short half-life in blood lead to a low-sensitivity detection of BNP, which is a challenge that has to be overcome. In this work, we propose a highly specific, highly sensitive T7 RNA polymerase-assisted clustered regularly interspaced short palindromic repeats (CRISPR)/Cas13a system to detect BNP via an electrochemiluminescence (ECL) sensing platform and incorporate exonuclease III (Exo III)-hairpin and dumbbell-shaped hybridization chain reaction (HCR) technologies. In this detection scheme, the ECL sensing platform possesses low background signal and high sensitivity. Firstly, the T7 promoter-initiated T7 RNA polymerase acts as a signal amplification technique to generate large amounts of RNAs that can activate CRISPR/Cas13a activity. Secondly, CRISPR/Cas13a is able to trans-cleave the surrounding trigger strand to produce DNA1. Thirdly, DNA1 is involved in the co-amplification reaction of Exo III and hairpin DNA, which subsequently triggers a dumbbell-shaped HCR technology. Eventually, a large number of Ru (II) molecules are inserted into the interstitial space of the dumbbell-shaped HCR to generate a strong ECL signal. The CRISPR/Cas13a possesses outstanding specificity for a single base and increased sensitivity. The tightly conformed dumbbell-shaped HCR provides higher sensitivity than the traditional linear HCR amplification technique. Ultimately, the clever combination of several amplification reactions enables the limit of detection (LOD) as low as 3.2 fg/mL. It showed promise for clinical sample testing, with recovery rates ranging from 98.4% to 103% in 5% human serum samples. This detection method offered a valuable tool for early HF detection, emphasizing the synergy of amplification strategies and specificity conferred by CRISPR/Cas13a technology.

Machine Learning-Assistant Colorimetric Sensor Arrays for Intelligent and Rapid Diagnosis of Urinary Tract Infection

Urinary tract infections (UTIs), which can lead to pyelonephritis, urosepsis, and even death, are among the most prevalent infectious diseases worldwide, with a notable increase in treatment costs due to the emergence of drug-resistant pathogens. Current diagnostic strategies for UTIs, such as urine culture and flow cytometry, require time-consuming protocols and expensive equipment. We present here a machine learning-assisted colorimetric sensor array based on recognition of ligand-functionalized Fe single-atom nanozymes (SANs) for the identification of microorganisms at the order, genus, and species levels. Colorimetric sensor arrays are built from the SAN Fe1-NC functionalized with four types of recognition ligands, generating unique microbial identification fingerprints. By integrating the colorimetric sensor arrays with a trained computational classification model, the platform can identify more than 10 microorganisms in UTI urine samples within 1 h. Diagnostic accuracy of up to 97% was achieved in 60 UTI clinical samples, holding great potential for translation into clinical practice applications.

PfAgo-Based Zika Virus Detection

As a mosquito-borne flavivirus, Zika virus (ZIKV) has been identified as a global health threat. The virus has been linked to severe congenital disabilities, including microcephaly and other congenital malformations, resulting in fatal intrauterine death. Therefore, developing sensitive and specific methods for the early detection and accurate diagnosis of the ZIKV is essential for controlling its spread and mitigating its impact on public health. Herein, we set up a novel nucleic acid detection system based on Pyrococcus furiosus Argonaute (PfAgo)-mediated nucleic acid detection, targeting the non-structural protein 5 (NS5) region of the ZIKV genome (abbreviated ZIKV-PAND). Without preamplification with the polymerase chain reaction (PCR), the minimum detection concentration (MDC) of ZIKV-PAND was about 10 nM. When introducing an amplification step, the MDC can be dramatically decreased to the aM level (8.3 aM), which is comparable to qRT-PCR assay (1.6 aM). In addition, the diagnostic findings from the analysis of simulated clinical samples or Zika virus samples using ZIKV-PAND show a complete agreement of 100% with qRT-PCR assays. This correlation can aid in the implementation of molecular testing for clinical diagnoses and the investigation of ZIKV infection on an epidemiological scale.