Virus short definition

In this case, symptoms arise as a result of opportunistic infections due to immunosuppression. JCPyV is a very common virus, infecting around half of the general population. JCPyV establishes a slow and possibly latent infection in kidney cells. It can reactivate in immunosuppressed individuals to cause progressive multifocal leukoencephalopathy, a brain disease with a high mortality rate that damages the white matter of the brain. Epidemiology is the study of how diseases, including those caused by viruses, spread throughout a population.

Epidemiologists are the detectives of the public health world: during a disease outbreak, they determine how a disease is being transmitted from person to person and establish control measures that interrupt the continued transmission of the pathogen.

Within a population, infections can be classified based upon their frequency of occurrence. A sporadic disease occurs infrequently and without a consistent pattern.

A single case of hantavirus in Nevada would be considered sporadic in nature. Endemic refers to the usual presence of a disease in a population at any given time. It is not necessarily a desired level, but it is the norm for a particular area. For example, during a normal September, 3. An epidemic occurs when there are clearly more cases of disease in a particular area than are expected during endemic periods. This is also referred to as an outbreak.

From August through January , people in 49 US states and the District of Columbia contracted enterovirus D68; only small numbers had been previously reported each year. When an epidemic spreads throughout several countries or the world, it is referred to as a pandemic. Assuming that diseases do not occur completely randomly, models are helpful in understanding how viral diseases are caused and spread.

The epidemiologic triad model consists of three factors: an external agent , a susceptible host , and an environment that brings the host and agent together. This can be represented as a triangle or as a balance Fig. The epidemiologic triad model is used to represent how infectious diseases are caused and spread. It illustrates that for infection to occur, an external agent and susceptible host must be brought together by the environment.

This can also be thought of as a balance between agent and host factors within an environment. In this model, agent refers to the pathogen and its characteristics that could affect its ability to be spread throughout a population. For instance, how is the pathogen transmitted? How stable is it in the environment? Does it have enhanced virulence factors?

Is it susceptible to current antiviral drugs? On the other hand, host refers to the human that may come into contact with the agent. The presence of the agent is required for disease to occur, but coming into contact with the agent does not necessarily mean that disease will occur in the host.

The environment refers to the extrinsic factors that affect whether or not the host will come into contact with the agent. These include socioeconomic factors proper sanitation, crowding, availability of health services , biological factors presence of vectors that transmit the agent, other animals that spread the virus , and physical factors climate, physical environment.

For disease to occur, the environment brings together the agent and a susceptible host. The specific details of the epidemiologic triad model can be elaborated upon by examining the factors found within the chain of infection : transmission of the agent to the host occurs when the agent leaves its reservoir through a portal of exit , is conveyed by a mode of transmission , and enters a susceptible host through a portal of entry Fig. The chain of infection represents that an infectious agent leaves its reservoir through a portal of exit, is conveyed by a mode of transmission, and enters a susceptible host through a portal of entry.

Epidemiologists identify and examine each step in order to institute control measures that prevent further transmission of the virus. Since viruses cannot reproduce outside of a host, humans are most often the reservoirs of human viruses. Some viruses replicate exclusively in humans, including smallpox or polio, while other viruses are able to infect humans and other animals.

An example is influenza A viruses, which can infect humans, waterfowl, pigs, and other animals. A zoonosis is an infectious disease that can be transmitted from an animal to a human. A carrier is a reservoir that can transmit the pathogen but shows no symptoms of the infection.

This can occur with healthy hosts that are asymptomatic, or because a person has been infected but is still within the incubation period of the illness, before symptoms appear. Similarly, people in the convalescent period may also be capable of transmitting the disease, even though their symptoms have subsided.

Within the chain of infection, the agent leaves its reservoir through a portal of exit. As described in detail in Section 5. The virus is conveyed through a mode of transmission , which can be through direct or indirect means. Direct transmission refers to the transfer of the virus by direct contact or droplet spread. Skin-to-skin contact, sexual intercourse, or kissing would be considered direct contact, while droplet spread includes the transmission of virions in respiratory droplets that are sneezed or coughed out of one person and immediately enter the respiratory tract of another person.

Indirect transmission , on the other hand, requires the presence of an intermediary between hosts. In comparison to droplet spread, which passes directly from one person to another, airborne transmission of viruses carried by dust or aerosolized particles that remain suspended in the air for long periods of time are considered indirect modes of transmission see Section 5.

Vehicles refer to nonliving physical substances—such as food, water, blood, or inanimate objects fomites —that can indirectly transmit virions. Vectors are living intermediaries that can also transmit viruses. Mosquitos, fleas, and ticks are examples of vectors. Since these are arthropods, the terms arbovirus is used to denote ar thropod- bo rne viruses. The agent virus is transmitted to and enters a subsequent host through a portal of entry. As described in Section 5.

The susceptibility of the host will depend upon those host factors described above, including age, sex, nutritional status, and immune status. Identifying the factors involved at each step within the chain of infection allows epidemiologists to devise control measures that interfere with the transmission of the virus from the reservoir to a susceptible host Table 5. The spread of a virus can be interrupted immediately by preventing the virus from leaving the infected individual.

Antiviral drugs are available against HIV, HSV, and certain strains of influenza, and the use of antiviral drugs can be effective in reducing viral load and transmission of the virus, even if the host is not cured of the disease. Those coming into direct contact with infected individuals, such as hospital employees, must also be diligent about handwashing and sterilization measures. For some zoonotic infections, infected animal hosts can be removed or relocated.

In other cases, infected patients can be isolated from other individuals to control the direct transmission of the virus. During the Ebola outbreak, infected individuals were quarantined in an attempt to prevent further transmission of the virus.

Other control measures attempt to interrupt the indirect transmission of the virus. For example, ventilation systems can be modified to prevent airborne transmission, waste water can be treated to kill viruses, clean drinking water can be provided, or insect spraying programs can be instituted to reduce vector populations. For viruses that are transmitted through the fecal—oral route, the environment can be rearranged to prevent transmission.

If the reservoir and environment cannot easily be modified, portals of entry can be protected to prevent infection of the host. Appropriate precautions and proper personal protective equipment PPE , such as gloves or safety glasses, can be used to protect portals of virus entry. Other physical barriers can be instituted: bed nets can protect infection by mosquitos, and wearing long pants and using insect repellent can prevent interaction with mosquitos, fleas, and ticks.

The epidemiologic triad model requires the interaction of the agent and host in an environment that brings the two into contact, but this does not mean that infection will necessarily occur. A susceptible host is required, and control measures to increase host immune defenses can interrupt the chain of infection.

Decreasing malnutrition can positively affect the immune system, but the majority of viruses that cause disease have evolved mechanisms to infect individuals with perfectly functional immune systems.

Vaccination is by far the most effective means of preventing susceptibility to these pathogens. Vaccination can protect the individual, but it can also protect a population through herd immunity Fig. If a large enough proportion of the population is vaccinated, then an infected individual may not come into contact with any susceptible hosts and the chain of infection will be terminated. Smallpox, caused by the variola virus, was eradicated from the human population by vaccinating any individuals that came into contact with an infected person.

Eventually there were no new susceptible hosts for the virus, and it died out. Herd immunity is effective in preventing epidemics, but a virus is still able to cause an outbreak if a specific population within the herd chooses to not vaccinate its individuals.

This also compromises the individuals within the herd that were unable to receive the vaccine due to medical reasons, such as allergy or immunosuppression.

A If a high proportion of the population is vaccinated against a pathogen, then transmission of the pathogen will soon cease due to lack of susceptible hosts. Note that herd immunity is able to protect those individuals who are unable to be vaccinated because they are too young or immunocompromised in blue. B If a population does not have high vaccination rates, however, then the pathogen continues to easily spread throughout the population.

An orderly examination of all the facts surrounding an outbreak is required for epidemiologists to accurately investigate the variables within the chain of infection. Epidemiologists also determine the morbidity rate of illness and mortality rate of death associated with an illness. The incidence of the disease refers to the number of new cases within a population during a specified time, while the prevalence of a disease refers to the total number of individuals with the disease at that time.

In other words, Before counting cases, however, an epidemiologist must determine what qualifies as a case of the illness. Epidemiological studies rely upon a case definition to determine whether or not a person has a particular disease. The case definition is a set of clinical and laboratory criteria that rely upon the symptoms the person presents with and the results of virus-specific blood tests. For a specific outbreak, limitations on the time and location may also be included within the case definition.

Nationally and internationally, use of a standard case definition allows for proper diagnosis and also ensures comparability among different hospitals and locations. Classification of results can categorize the case as suspected, probable, or confirmed Fig. A case definition is a set of clinical and laboratory criteria to classify an potentially infected person. In this CDC case definition, a probable case of measles virus is an illness that meets the clinical criteria but has not been confirmed with laboratory testing.

A confirmed case occurs when a person has a rash and one of several confirmatory laboratory tests showing the presence of the measles virus or a recent immune response against it. Case definitions can change over time and often do so as more information about the illness becomes available. When trying to determine the specific cause of an outbreak, however, epidemiologists employ a strict case definition that will only confirm those infected with the particular pathogen.

For instance, an epidemiologist studying the cause of a Norwalk virus outbreak, which is a fecal—oral disease that causes diarrhea, will not want to include diarrhea as the only requirement—many other infectious and noninfectious diseases also cause diarrhea as a symptom.

Loose and strict case definitions may overestimate and underestimate the total number of cases, respectively, but are necessary for containing serious viral outbreaks or determining the definitive source of an infection. In the United States and in many countries around the world, state and national public health departments must be notified when a patient is diagnosed with certain infectious and noninfectious conditions.

This surveillance system is used to monitor disease trends, identify populations at high risk, formulate control measures, and create public health policies. The list of notifiable viral diseases is listed in Table 5. These are also known as epidemiological variables. Epidemiological studies are divided into descriptive and analytic studies.

Both types of study start with the agent, defined by the case definition, and identify the who, where, and when host, location, and time. In addition, analytic studies try to determine the cause and transmission. Time refers not only to the hour and minute of the day, but also to the season and time of year.

Some diseases occur more frequently during a particular time of the year; for example, influenza viruses peak during the winter months Fig.

In contrast, the incidence of diseases transmitted by mosquitoes, such as West Nile virus or eastern equine encephalitis virus, is higher at the end of the summer when mosquito populations are highest.

Determining the typical yearly or seasonal pattern of viral infections is important in creating a baseline that can be used to compare future occurrence of the disease or monitor the effectiveness of control measures.

Graphing the time of the incident versus the occurrence of symptoms can also be useful in assessing the incubation period of the virus, which can be helpful in identifying the specific virus. Many epidemiological variables affect the transmission of viral infection and are worthwhile to consider while analyzing disease transmission. This graph shows that in —15, laboratory-confirmed cases of influenza were at their highest in the United States from November to March, peaking in December of Place refers to the local location of the case as well as the larger geographic location.

A gastrointestinal virus outbreak may occur at a restaurant the place , suggesting that the food may have been involved in the transmission of the virus. A person presenting with neurological manifestations in a rural location may suggest different viruses than in a big city—for example, rabies transmission is much more likely in areas where wildlife is prevalent. It would also be important to note that a patient presenting with hepatitis recently traveled to a remote location in South America.

This also highlights that, because of viral incubation periods, the location and timing of symptoms does not always correlate with when infection occurred Fig. A clearer correlation with place could be determined when looking at the location to which each individual recently traveled: the majority of infected individuals traveled to areas of China, Singapore, or Taiwan B. Details concerning the person host can affect the chain of infection and are important to consider in epidemiological studies.

Almost every health-related event varies with age because it is a factor in exposure, immune status, and physiological response. Influenza A virus causes higher morbidity and mortality rates in young children and the elderly. Many other personal attributes can contribute to infection.

The sex of the individual can sometimes be a consideration—cervical cancer caused by HPV will not occur in men—and being part of different genetic, cultural, or social groups can contribute to the exposure of an individual to a particular virus.

The socioeconomic status of an individual income, education, and occupation can also play a role in exposure to the virus and access to medical care. Descriptive studies are effective in chronicling patterns and developing hypotheses as to the cause of an illness or outbreak. A hallmark of analytic studies is the presence of a comparison control group that can be used to generate baseline data to which the outbreak or illness can be compared.

With a comparison group, statistical analyses can be performed to determine a cause with good certainty. Epidemiological studies fall into two general categories: experimental and observational. Experimental studies are planned, controlled studies; a clinical trial to test new vaccines that enrolls participants into the study, randomly assigns them into one of three groups vaccine A, vaccine B, or placebo , and then gathers data is an experimental study.

As the name implies, observational studies involve the observation of subjects and subsequent recording of data. In comparison to experimental studies, the epidemiologist does not have any influence over what exposure the participant receives.

Observational studies are more common in epidemiology than experimental studies. Observational studies fall into three categories: cohort studies , case-control studies , and cross-sectional studies. Cohort studies are similar to experimental studies in that two groups are compared in real time, except that, being an observational study, the epidemiologists do not assign participants to cohorts , or groups.

Instead, they allow the natural course of things to proceed, tracking whether or not the two cohorts have different results. An example of a cohort study would be to observe consistent users of hookah pipes, a water pipe used to smoke flavored tobacco. One cohort of participants uses individual disposable plastic mouthpieces on their pipes, while the other group uses the attached metal mouthpiece.

This study might observe whether HSV-1 transmission is more common in those using the metal, shared mouthpieces compared to those who each have their own disposable plastic mouthpiece.

If this were an experimental study, the epidemiologists would have assigned each participant to a specific group, either the group that uses the shared mouthpiece or the disposable plastic mouthpiece. A second type of observational study, the case-control study, is always retrospective, meaning that it analyzes past events. A case-control study happens after an event for example, a viral outbreak has occurred.

An example of a case-control study would involve the infection of several people with hepatitis C virus at a tattoo parlor. After noting that all the case patients received their tattoos from one tattoo artist, epidemiologists retroactively enrolled a group of people that received tattoos from the other tattoo artist at the parlor. The control group allows them to have a baseline group to determine the typical infection rate. In this case, it was determined that one of the tattoo artists, but not the other, was improperly sterilizing tattoo equipment.

The final type of observational study is the cross-sectional study. For example, a cross-sectional study might find that a high proportion of those individuals that have liver scarring have hepatitis C infection.

Although the obvious conclusion seems to be that the liver scarring must be caused by the virus, it is also possible that the scarring makes the liver more susceptible to hepatitis C infection, and that is why these individuals have high rates of both liver scarring and hepatitis C. Correlation does not equal causation! Therefore, a cross-sectional study is not effective as an analytic study, but they are used routinely for descriptive studies.

Since both the cohort and case-control study have comparison groups, they would be considered analytic studies. Cross-sectional studies often do not have a comparison group as a control and are therefore most often carried out as descriptive studies. Section 5. Make a table listing each portal of entry. What defenses does the host have at each location and how are viruses able to successfully bypass them?

Norwalk virus causes significant morbidity in developed nations, despite that these countries have clean water supplies. How do you think the virus is transmitted, and why it is so successful? Describe the architecture of the skin and how viruses gain access to each layer and the subcutaneous tissue. Describe how different factors could affect the inactivation of virions within the environment.

Your friend walks into class, still sniffling occasionally from a respiratory viral illness. Make a list of the three major aspects of the epidemiologic triad model and what factors could affect each aspect in promoting infection. Create a list of control measures that interfere with each variable within the chain of infection. Design a case-control study that attempts to determine the precise food product that was the cause of a viral gastrointestinal illness.

National Center for Biotechnology Information , U. Essential Human Virology. Published online May 6. Jennifer Louten. Author information Copyright and License information Disclaimer.

Elsevier hereby grants permission to make all its COVIDrelated research that is available on the COVID resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source.

Abstract For transmission of a virus to occur, a virus must enter a host through a portal of entry, replicate or disseminate within the host, and be transmitted to a new host through a portal of exit. Portals of Virus Entry To establish infection, a virus must come in contact with host cells that are susceptible and permissive to infection. Open in a separate window. Figure 5.

Common portals of virus entry. Table 5. Respiratory Tract The respiratory tract is the most common portal of entry for viruses into the human body. The respiratory tract. Gastrointestinal Tract The human digestive or gastrointestinal tract is a hollow tube that stretches from the oral cavity mouth to the anus Fig.

The gastrointestinal tract. Study Break. Genital Tract The genital tract refers to the organs that are involved in reproduction. Skin The skin is a unique organ, a covering to the body that creates 1.

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The maximum amount of data transmitted over an internet connection in a given amount of time. Antivirus Definition Software that is created specifically to help detect, prevent and remove malware malicious software. More from Merriam-Webster on coronavirus Britannica. Get Word of the Day daily email! Test Your Vocabulary. Test your visual vocabulary with our question challenge! A daily challenge for crossword fanatics. Love words? Need even more definitions?

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