Microbiology and Immunology

Epidemiology WORLD OF MICROBIOLOGY AND IMMUNOLOGY

198

•

A final example of a twentieth century viral epidemic is

that caused by the Hanta virus. The virus causes a respiratory

malady that can swiftly overwhelm and kill the patient. The

virus is normally resident on certain species of mouse. In the

mid-1990s, an epidemic of Hanta virus syndrome occurred in

native populations in the Arizona and New Mexico areas of

the United States west. As with other viral epidemics, the epi-

demic faded away as quickly as it had emerged. However,

exposure of someone to the mouse host or to dried material

containing the virus particles can just as quickly fuel another

epidemic.

Given their history, it seems unlikely viral epidemics

will be eliminated. While certain types of viral agents will be

defeated, mainly by the development of effective vaccines and

the undertaking of a worldwide vaccinationprogram (e.g.,

smallpox), other viral diseases will continue to plague

mankind.

See alsoAIDS; Hemorrhagic fevers and diseases; Virology

EEpidemiologyPIDEMIOLOGY

Epidemiology is the study of the various factors that influence

the occurrence, distribution, prevention, and control of dis-

ease, injury, and other health-related events in a defined

human population. By the application of various analytical

techniques including mathematical analysis of the data, the

probable cause of an infectious outbreak can be pinpointed.

This connection between epidemiology and infection makes

microorganismsan important facet of epidemiology.

Epidemiology and genetics are two distinct disciplines

that converge into a new field of human science. Genetic epi-

demiology, a broad term used for the study of genetics and

inheritance of disease, is a science that deals with origin, dis-

tribution, and control of disease in groups of related individu-

als, as well as inherited causes of diseases in populations. In

particular, genetic epidemiology focuses on the role of genetic

factors and their interaction with environmental factors in the

occurrence of disease. This area of epidemiology is also

known as molecular epidemiology.

Much information can come from molecular epidemiol-

ogy even in the exact genetic cause of the malady is not

known. For example, the identification of a malady in genera-

tions of related people can trace the genetic characteristic, and

even help identify the original source of the trait. This

approach is commonly referred to as genetic screening. The

knowledge of why a particular malady appears in certain peo-

ple, or why such people are more prone to a microbial infec-

tion than other members of the population, can reveal much

about the nature of the disease in the absence of the actual

genewhose defect causes the disease.

Molecular epidemiology has been used to trace the

cause of bacterial, viral, and parasitic diseases. This knowl-

edge is valuable in developing a strategy to prevent further

outbreaks of the microbial illness, since the probable source of

a disease can be identified.

Furthermore, in the era of the use of biological weapons

by individuals, organizations, and governments, epidemiolog-

ical studies of the effect of exposure to infectious microbes has

become more urgently important. Knowledge of the effect of

a bioweapon on the battlefield may not extend to the civilian

population that might also be secondarily affected by the

weapons. Thus, epidemiology is an important tool in identify-

ing and tracing the course of an infection.

The origin of a genetic disease, or the genetic defect that

renders someone more susceptible to an infection (e.g., cystic

fibrosis), can involve a single gene or can be more complex,

involving more than one gene. The ability to sort through the

information and the interplay of various environmental and

genetic factors to approach an answer to the source of a dis-

ease outbreak, for example, requires sophisticated analytical

tools and personnel.

Aided by advances in computer technology, scientists

develop complex mathematical formulas for the analysis of

genetic models, the description of the transmission of the dis-

ease, and genetic-environmental interactions. Sophisticated

mathematical techniques are now used for assessing classifi-

cation, diagnosis, prognosis and treatment of many genetic

disorders. Strategies of analysis include population study and

family study. Population study must be considered as a broad

and reliable study with an impact on public healthprograms.

They evaluate the distribution and the determinants of genetic

traits. Family study approaches are more specific, and are usu-

ally confirmed by other independent observations. By means

of several statistical tools, genetic epidemiologic studies eval-

uate risk factors, inheritance and possible models of inheri-

tance. Different kinds of studies are based upon the number of

people who participate and the method of sample collection

(i.e., at the time of an outbreak or after an outbreak has

occurred). A challenge for the investigator is to achieve a

result able to be applied with as low a bias as possible to the

general population. In other words, the goal of an epidemio-

logical study of an infectious outbreak is to make the results

from a few individuals applicable to the whole population.

Such analytical tools and trained personnel are associ-

ated more with the developed world, in the sense that expen-

sive analytical equipment and chemicals, and highly trained

personnel are required. However, efforts from the developed

world have made such resources available to under-developed

regions. For example, the response of agencies such as the

World Health Organizationto outbreaks of hemorrhagic fevers

that occur in underdeveloped regions of Africa can include

molecular epidemiologists.

A fundamental underpinning of infectious epidemiol-

ogy is the confirmation that a disease outbreak has occurred.

Once this is done, the disease is followed with time. The pat-

tern of appearance of cases of the disease can be tracked by

developing what is known as an epidemic curve. This infor-

mation is vital in distinguishing a natural outbreak from a

deliberate and hostile act, for example. In a natural outbreak

the number of cases increases over time to a peak, after which

the cases subside as immunitydevelops in the population. A

deliberate release of organisms will be evident as a sudden

appearance of a large number of cases at the same time.

womi_E 5/6/03 2:12 PM Page 198