signs or symptoms. These disease-causing agents can even be contagious during
the recovery period.
When a person becomes symptomatic(feeling sick), the individual seeks
medical attention and receives treatment. In many cases, however, diseases are
spread from a person with subclinical findings—the symptoms are very mild and
not recognizable. These individuals can spread the disease to a healthy person.
Asymptomaticcases are a problem because infected people can infect other indi-
viduals without knowing they are infected. These individuals that are “carriers”
of disease are called disease carriers.
Carriers of disease are classified as:- Subclinical carriers. These individuals never develop clinical symptoms
of the disease. - Incubatory carriers.These individuals transmit the disease before be-
coming symptomatic. - Convalescent carriersare individuals are recovering from the disease; how-
ever, they can still infect other people. - Chronic carriers. These individuals develop chronic infections and trans-
mit the infection for long periods of time.
ANIMAL RESERVOIRS
Many microorganisms can infect both humans and animals. Many of these dis-
ease-causing agents use animals as reservoirs of infection to infect humans.
Apes and monkeys are good examples of animals that serve as reservoirs for
human infection because they are physiologically similar to humans. When an
animal infects humans, the humans can also serve as reservoirs for the infection.
Adisease that is transmitted from domestic and wild animals to humans is
called zoonosis.Two examples of zoonoses are anthrax, which is a bacterial dis-
ease that causes infection in dogs, cats, cattle, and other domestic animals.
Humans become infected with direct contact with the animals, their wool, or
hides, contaminated soil, inhalation of spores, and ingestion of meat or milk.
Another is rabies, a virus that infects dogs, cats, skunks, wolves, and bats.
Humans become infected throughinfected saliva in bite wounds. Humans and
domestic animals can also be reservoirs for wild animals.NONLIVING RESERVOIRS
Examples of nonliving reservoirs are water and soil. Soil is a good reservoir for
the bacterium Clostridium tetani, which causes the disease tetanus. Conta-(^200) CHAPTER 13 Epidemiology and Disease