skin (no cuts or abrasions), mucous membranes, a good cough reflex, normal
gastric juices, and normal bacterial flora.
If a microorganism penetrates these defenses the development of a disease
process begins. First there may be an incubation period. This is the time between
the initial exposure and start of the infection to the first appearance of signs and
the feeling of symptoms. Different microorganisms have different incubation
periods. An examples is the Epstein-Barr virus, which causes infectious mono-
nucleosis and has an incubation period of two to six weeks.
The varicella-zoster virus, which causes varicella (chickenpox), has an incu-
bation period of two weeks. The human immunodeficiency virus, the virus that
causes AIDS, has an incubation period of 7 to 11 years. During this phase, the
disease can be spread from the infected individual to a non-infected individual.
The prodromal periodfollows the incubation period. This period presents
with mild symptoms.
The period of illnessis the acute phase of the disease. Here the individual
presents with signs and symptoms of the disease. Signsare objective findings
that an observer or physician can see. These are physical changes that can be
measured. Examples of signs are fever, skin color or lesions, blood pressure,
inflammation, and paralysis.
Symptoms are subjective and cannot be seen by an observer. Symptoms pres-
ent as changes in bodily functions, such as pain, numbness, chills, general fatigue,
or gastrointestinal discomfort. It is in this period of the disease where white blood
cells may increase and the individual’s immune system responds to combat the
disease-causing pathogen. If the individual’s defense mechanism of the immune
system does not successfully overcome the disease or if the disease is not treated
properly, the person can die.
During the period of decline, the individual’s signs and symptoms subside
and the person feels better. This period may take 24 hours to several days.
During this time, the individual is prone to secondary infections.
The period of convalescenceis the phase where recovery has occurred. The
body regains strength and is returned to a state of normality. During this phase,
infection can also be spread.Epidemiological Studies
Epidemiological studies began in 1855 with the work of English physician John
Snow. Snow conducted studies relating to the cholera outbreak in London,
England. Snow, through careful analysis of deaths related to cholera, case histo-
ries of victims, and interviews with survivors, traced the source of the epidemic(^204) CHAPTER 13 Epidemiology and Disease