The most common causes include viral infection and noninfectious causes such as lead poisoning.

Attenuated.

Having decreased virulence; used especially for live virus vaccines. Attenuation is achieved either through selection of less virulent mutants or through physiologic alteration by exposure to unfavorable conditions.

Attributable risk (exposed).

The rate of a disease or other outcome in exposed individuals that can be attributed to the exposure. Synonymous with risk difference and excess risk. This measure is estimated by subtracting the rate of the outcome (usually incidence or mortality) among unexposed individuals from the rate among exposed individuals. It is assumed that causes other than the one under investigation have had equal effects on exposed and unexposed groups and that the effects of different causes are additive. This term is often used, incorrectly, to denote the attributable fraction among exposed individuals in the population.

B

Bayes theorem.

A theorem in probability theory. In epidemiology, it is used to obtain the probability of disease in a group of people with some characteristic on the basis of the overall rate of that disease (the prior probability of disease) and of the likelihoods of that characteristic in healthy and diseased individuals. The most familiar application is in clinical decision analysis where it is used for estimating the probability of a particular diagnosis given the appearance of some symptom or test result.

Bias.

Deviation of results or inferences from the truth, or processes leading to such deviation. Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth. Not to be confused with prejudice or partisan point of view, as is the conventional usage.

Brachial neuritis.

Also known as brachial plexus neuropathy or as neuralgic amyotrophy. A neuropathy which presents as a deep, steady, often severe aching pain in the shoulder and upper arm. See Chapter 3.

C

Case-control study

(Syn: case-comparison study). A controlled observational study that starts with the identification of persons with the disease or condition (adverse event) of interest and a suitable control (comparison) group of persons without the disease. The relation of an attribute (e.g., immunization) to the disease is examined by comparing the diseased and nondiseased groups with regard to how frequently the attribute is present, or if quantitative, the levels of the attribute, in each of the groups.

Cohort study.

(Syn: prospective, follow-up study). A controlled observational study in which subsets of a defined population can be identified who



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