Viruses are extremely small (.02-.2 micron), non-cellular particles consisting of short strands of RNA or DNA surrounded by a protein coat that usually assumes a simple geometric shape. They are non-living because they don't have the cellular components necessary to synthesize their own proteins or conduct metabolism to generate energy. Consequently, they can't replicate by themselves; they need to attach and gain access to a host cell in which their DNA or RNA utilizes the host cells cellular machinery to synthesis more of its DNA or RNA and proteins for its coat. These self-assemble into new virus particles that are released when the cell ruptures. They are almost all pathogenic (cause disease) and cause disease by destroying cells after replication and by provoking an inflammatory response in the host. Viruses are unaffected by antibiotics.
State laws require that certain diseases be reported in order to identify health problems that require attention and to monitor changes in disease frequency over time. In Massachusetts the state DPH has the authority to determine which diseases must be reported and how. State law specifically lists these disease and stipulates that household members, physicians and other health care providers must promptly report cases to the local board of health; the local board is then required to report this information (including the case's name, date of birth, age, sex, address and disease) to the state Department of Public Health (DPH). Similarly, all laboratories, including those outside of Massachusetts, performing examinations on any specimens derived from Massachusetts residents that yield evidence of reportable infections must be reported to the Massachusetts DPH. These state laws and regulations provide the basis for an ongoing system of disease surveillance.
A general term for tests that measure the level of specific antibodies, such as IgM anti-HAV; elevated blood levels of IgM anti-HAV, indicate recent infection with hepatitis A.
A bias that can be introduced into an epidemiologic study if the mechanism used to enroll subjects differs in a way that is related to the association being tested. For example, in a study testing the association between socioeconomic status and risk of cancer of the cervix, it is important to select cases and controls by a similar mechanism. If controls were more or less likely to be selected for inclusion into the study because of socioeconomic factors, a bias would be introduced.
Sampling error refers to the extent to which estimates based on samples may be inaccurate just based on variability from one sample to the next (the luck of the draw). Intuitively, sampling errors are potentially large with small sample size, and become smaller as the sample size increases.
One generally doesn’t measure all members of a population when trying to characterize it or test hypotheses about factors associated with disease. Instead, one selects a sample of subject and makes estimates of population parameters (e.g. disease frequency, mean body weight, etc.) based on samples.
A cohort study is an epidemiologic study in which a disease-free cohort of subjects is divided into two or more comparison groups based on their exposure to a putative risk factor; the investigators then measure and compare the subsequent incidence of disease among the exposure groups. A retrospective cohort study is one in which exposure status and disease outcome have already occurred, but the investigators go back in time to identify the cohort and classify subjects with respect to exposure status; they then measure and compare the subsequent incidence of disease among the comparison groups. In a prospective study, the investigators must follow the group into the future for a period of time in order measure the incidence of disease in the comparison groups.
State laws require that certain diseases be reported in order to identify health problems that require attention and to monitor changes in disease frequency over time. In Massachusetts the state DPH has the authority to determine which diseases must be reported and how. State law specifically lists these disease and stipulates that household members, physicians and other health care providers must promptly report cases to the local board of health; the local board is then required to report this information (including the case's name, date of birth, age, sex, address and disease) to the state Department of Public Health (DPH). Similarly, all laboratories, including those outside of Massachusetts, performing examinations on any specimens derived from Massachusetts residents that yield evidence of reportable infections must be reported to the Massachusetts DPH. These state laws and regulations provide the basis for an ongoing system of disease surveillance.
A measure of the strength of association between an exposure and a disease outcome calculated by expressing the incidence of disease in exposed and unexposed individuals as a ratio. For example, for the exposure between heavy cigarette smoking and lung cancer, some studies have estimated relative risk to be about 20, suggesting that those who smoke cigarettes heavily have twenty times the risk of lung cancer compared to non-smokers.
The proportion of a population that has disease at a specific point in time.
Also known as “police power”. When laws are enacted and regulations are promulgated to protect the health of the public, fundamental human rights may be compromised. Legitimate questions might be asked regarding whether a law or regulation addresses a legitimate state concern and whether the law or regulation achieves its intended purpose. Decisions regarding the power of the state with respect to public health laws hinge on an assessment of the health risks and the extent to which laws and regulations protect the public’s health.
An outbreak in which persons are exposed to a common source of infection over a fairly brief period of time. Outbreaks of this type tend have characteristic epidemic curves with a sharp increase and a sharp decrease.
A propagated source epidemic is one in which one or more of the first wave of cases serves as a source of infection for subsequent cases and those subsequent cases, in turn, serve as sources for later cases. The resulting epidemic curve usually contains a series of successively larger peaks, which are one incubation apart with an increasingly large number of cases, until the pool of susceptible persons is exhausted or control measures are implemented.
The period during which an infected individual can pass a disease on to others. For example, hepatitis A is spread primarily via the fecal oral route.
In an experimental or epidemiological study, the p-value is the probability of seeing the observed differences between the groups (or greater differences), if the groups were really not different (i.e., if the null hypothesis were true).
The measure of association in a case-control study. In a case-control study one cannot calculate relative risk, because you can’t measure incidence (since you are comparing people who have the disease to people who do not have it). Instead, one measures the association between exposure and disease by calculating an odds ratio. This is literally the odds that a case has the exposure of interest, divided by the odds that a control subject has the exposure. However, provided that the outcome of interest is relatively uncommon (e.g. lung cancer), the odds ratio provides a legitimate estimate of relative risk and can be interpreted as if it were a relative risk.
The value associated with no effect. For example, in a cohort study association is expressed by calculating either relative risk or the risk difference. A relative risk equal to 1.0 would indicate no difference in risk between the two groups being compared, and this would be the null value. Similarly, if there were no difference between the two groups, the risk difference would be zero, so this would be the null value for the risk difference. In a case-control study, an odds ratio equal to 1.0 is the null value, because it suggests that there is no difference in exposure status between the groups being compared.
A bias that results from errors in how subjects are classified with respect to either their exposures or their outcomes. For example, if a certain percentage of smokers were incorrectly classified as non-smokers, it could bias the estimate of association between smoking and heart disease.
A blood sample can be tested for levels of certain enzymes produced in liver cells. Blood levels of these enzymes are generally low, and elevated levels indicate damage to liver cells (liver cells break apart when damaged and release enzymes into the blood stream). Elevated liver function tests might, for example, indicate liver infection or exposure to toxic drugs or chemicals.
In an outbreak investigation a line listing is a summary of the characteristics of individuals who became ill. Each case is listed on a separate line, and their characteristics (e.g. date of onset of disease, age, gender, occupation, etc.) are summarized in columns.
When information for an epidemiological study is collected by interviewing subjects, the results may be biased by conscious or unconscious differences in how subjects are interviewed. For example, in a case-control study comparing subject with heart disease (cases) to a comparison group without heart disease (controls), the interviewer might probe more cases carefully about their past use of tobacco. In this case the association between tobacco and heart disease might be exaggerated.
Immune globulin (IG) is a sterile preparation of concentrated antibodies (immunoglobulins) made from pooled human plasma. IG provides passive immunity that only lasts for 3-5 months. If an individual is exposed the hepatitis A, the likelihood that they will become infected can be reduced by injecting them with IG.
This is an abbreviation for immunoglobulin M antibodies to the hepatitis A virus. Elevated levels of this antibody indicated recent infection with hepatitis A. A positive test for IgM anti-HAV clinches the diagnosis of acute infection with hepatitis A. After recovery, the IgM gradually declines, but anti-HAV IgG increases. IgG is another one of the five major classes of antibodies. In people who have recovered from hepatitis A or been vaccinated for it, anti-HAV IgG confers longer lasting immunity against hepatitis A infection.
The incubation period is the time period between infection and development of symptoms. For hepatitis A the average incubation period is about 28 days, with a range of 15-50 days.
The frequency with which new cases of disease occur in a population at risk.
A liver disease cause by the hepatitis A virus, which is most commonly transmitted via the fecal-oral route.
HIPPA is the Health Insurance Portability and Accountability Act. This is a federal law designed to safeguard your privacy with respect to health and medical information. HIPAA gives individuals specific rights with respect to their health information and sets limits on who has access to your health information.
A mechanism of disease transmission in which infective agents are shed into the stool of infected persons and are then ingested by another individual. This may result in a variety of ways, e.g. from drinking water contaminated with feces, from an infected child in a daycare setting contaminating toys which subsequently finding their way into the mouths of other children, or from an infected food worker who does not follow proper hygienic procedures (handwashing and/or glove use).
A graph that illustrates the rise and fall of new cases of a disease over time. The characteristics of an epidemic curve provide clues that may be helpful in identifying the source.
An epidemic (or outbreak) is said to occur when there is an increase in the observed number of cases above what normally is seen in a given location. The number of cases required to indicate an outbreak varies according to the agent, the location, and the population characteristics (size, previous exposure and level of immunity to the disease). For rare diseases (e.g. smallpox, botulism, rabies, polio) just a single cases qualifies as an outbreak that merits investigation. Information regarding the baseline frequency of disease is obtained from surveillance data. For example, Massachusetts has a list of disease conditions that must be reported (by law) either to the Massachusetts Department of Health or to the local health department.
There are many types of confidence intervals, but in the context of an outbreak investigation, one is primarily interested in the 95% confidence interval for a relative risk or an odds ratio. In a cohort study one measures association by calculating a relative risk. Since this is an estimate, based on a sample, one might calculate the 95% confidence interval, i.e. the range within which the true relative risk lies with 95% confidence. Similarly, in a case-control study association is measured by calculating an odds ratio, and one can also calculate the 95%
A continuous common source epidemic is one in which exposure to the source is prolonged over an extended period of time and may occur over more than one incubation period. An example, would be a prolonged outbreak of cholera due to a water supply contaminated with fecal waste.
A criteria by which one decides whether an individual is a disease “case” or not. Case definitions may vary in their specificity and sensitivity, depending upon whether one wants to capture all possible cases or a subset of individuals who clearly have the disease of interest. The definition used for an epidemiologic study may not be the same as that used by a clinician involved in the treatment of the disease. In 1990 the CDC published case definitions for a wide variety of diseases in order to provide more a more uniform standard for reporting.
An epidemiologic study in which a group of subjects with a specific disease (cases) are compared to a comparison group of non-disease subjects (controls) which respect to the likelihood of having had a given exposure. The association between exposure and disease is expressed as the odds ratio. (See below.)