Biology Reference
In-Depth Information
which the pathogenic microbe begins to multiply. The infected host
shows no symptoms at all during this period, which usually lasts from
several days to several weeks, but may be as short as a few hours or as
long as 10 or 12 years. The length of the incubation period is
characteristic of the particular infectious organism, within limits
determined by the health of the host and the route of infection. The
healthier the host and the longer the path the infecting organism must
travel, the longer the incubation period. If the host is healthy, nonspecific
defense mechanisms may be able to defeat some of the infecting
organisms, thus slowing the progress of the infection. If the path
traveled by the infecting organism is longer, it will take more time for
the infection to occur because each step in the path (for example, from
the mouth, through the gastrointestinal tract, and then into the
bloodstream in the case of some Salmonella species) will require some
amount of time to occur.
The host begins to feel ill during the prodromal period or prodromium. This
is a short period of mild symptoms, which may be difficult to characterize
as anything other than ''not feeling quite right.'' The prodromium is
rapidly followed by the period of illness or period of invasion, characterized
by the most rapid reproduction of the pathogen. The host develops
unmistakable symptoms, which may be quite severe. If the disease is
serious and the immune response is weak, delayed, or absent, the host
may die during this phase. The peak of this phase is called the fastigium.
If all goes well, however, the immune system will begin to bring the
infection under control, and the infection will enter the period of decline.
The host will begin to get better and enter the convalescent period,in
which the immune system elements (the antibodies and cytotoxic or
killer T cells) will be actively targeting and destroying the remaining
infectious agents. With many diseases, the host will acquire long-lasting
resistance to the disease, such that when she encounters the pathogen
again, her immune system will mount such a rapid and strong
reaction that an infection will not be established, and the host will not
even realize that she has encountered the pathogen again. This resistance
to recurring infection is called immunity.
For countless years, human beings had no choice but to suffer through
the above course of infection and, if they were fortunate, survive.
A major breakthrough was made in 1798, when the British physician
Edward Jenner (1749-1823) developed a vaccine against smallpox by
inoculating people with tissue from individuals infected with cowpox.
Even Lord Byron, in his typical ironic manner, praised this great
achievement:
With it the Doctor paid off an old pox
By borrowing a new one from an ox
...
Don Juan, Canto the 1st, CXXIX.
