CBC News Online | April 15, 2003 | Updated April 16, 2003
An infected person who is coughing and sneezing is the most obvious sign of SARS. But beyond the individual level, diseases spread through populations. The field of epidemiology studies the occurrence, spread and control of infectious disease in populations.
Prevalence: the proportion of diseased individuals in a population at any one time.
Incidence: the number of diseased individuals in a population at risk.
Epidemic: a disease that occurs in an unusually high number of individuals in a community at the same time. The U.S. Centers for Disease Control says that to epidemiologists "epidemic" and "outbreak" basically mean the same thing, and "outbreak" is often used to avoid sensationalism. SARS is considered an epidemic.
Pandemic: a very widespread, often global, disease.
Endemic: a disease that is constantly present, usually in low numbers in a population.
Quarantine: limiting the freedom of movement of individuals with active infections to prevent a disease from spreading to other people in the population. The length of a quarantine is the longest period of communicability of a disease. The term is based on the Latin word for 40. During the Great Plague, people were secluded for 40 days.
Isolation: is used for unusually infectious diseases in hospitals or health care facilities.
Community spread: random cases that can't be traced back to the first infected person. Health officials in Ontario say there hasn't been a widespread community spread of SARS.
Epidemiology is the study of how disease occurs in groups of people how it spreads and why. There are two types of epidemiology :
- Classical or statistical epidemiology looks at connections between disease agents viruses, bacteria or environmental factors and the people affected
- Ecological epidemiology looks at the interaction between a host and an infectious agent in the environment
An example of classical epidemiology is the use of statistics to tie lung cancer to cigarette smoking. Ecological epidemiology was used to find the connection between legionnaires' disease and bacteria that lived in air conditioners.
The first thing an epidemiologist looks for is the "population at risk," the people who could be counted as cases or potential cases, if they have the disease or condition the epidemiologist is looking at.
The problem facing epidemiologists is defining the population. It can't be limited to people who are currently sick. An example is HIV/AIDS. The first reported cases in North America were among gay men. First figures showed that 15 men were infected for every woman. But new populations were soon found in the western world, including intravenous drug users, people who had received transfusions of untreated blood, and some babies who were infected in the womb. It was soon found that HIV/AIDS was epidemic in central Africa, that transmission there was heterosexual and that the ratio of male to female infections was one-to-one.
The shape of the epidemic curves helps determine its likely origin. In a common source outbreak, such as from contaminated food or water source (e.g. Walkerton), the curve rises sharply and then declines rapidly. In a person-to-person epidemic, the curve rises and falls more slowly over several incubation periods (e.g. SARS).
Usually epidemiologists track a smaller sample population, paying special attention to the characteristics of the population what makes them stand out, what makes them susceptible.
An epidemiological study looks at a number of indicators:
- Time: exposure must precede the onset of symptoms.
- Consistency: the same effect is seen in independent and varied studies. (Note: The epidemiology of SARS is not yet consistent because the coronavirus is not found in all patients.)
- Dose response: the greater the exposure, the greater the effect on health. An example would be smoking: the more smoking done, the greater chance of lung cancer.
- Verification: the diagnosis must be verified to eliminate other possible causes.
- Characteristics of the patient: what it is about the patient that makes them susceptible. Epidemiologists work to find a common experience in patients and to also look for some experiences not shared by those who are disease free.
There are four types of epidemiological studies:
- Cohort. A cohort study follows a large group of usually healthy people to see who is vulnerable to a disease or condition. Different populations and exposures are measured. These studies work best over a long period of time.
- Case-Control. A case-control study looks at the affected population and tries to work out a cause. This works best for rare diseases and can be affected by conscious or unconscious bias among those doing the study.
- Occupational. This kind of study picks people who are in one occupation to find out if that occupation is more susceptible to a disease, usually from an environmental cause. Those doing this type of study have to be careful because levels of exposure are often different among people in a single occupation.
- Cross-sectional. A cross-sectional study compares different groups in terms of their exposure to an agent and the state of their health. The problem with this type of study is that it's usually a snapshot in time and may not take into account all factors involved. Often used when a neighbourhood reports a "cluster" of a disease or condition.
Sources: Centers for Disease Control and Prevention, An Introduction to Epidemiology
Biology of Microorganisms, seventh edition by Thomas Brock, Michael Madigan, John Martinko and Jack Parker, Prentice-Hall Inc. 1994