cysts that are much more resistant to chemical
agents or heat than the unprotected organisms.
Amoebiasis tends to come on slowly and
produce only mild diarrhea at first, even
though later effects from Amoebae can be dis-
astrous. Many individuals harboring these or-
ganisms may be unaware that they have an
infestation.
Although giardiasis has been blamed for a
variety of awful diarrheal disorders, over half
of the individuals exposed to the organism do
not develop infestations. Of those who are in-
fested, only about one-fourth develop symp-
toms. Typical symptoms appear about a week
after infestation and consist of three or four
soft stools daily. Feelings of bloating, in-
creased flatulence, and foul-smelling eructa-
tions ("rotten egg burps") are common, but for
most individuals the condition is more of a
nuisance than it is incapacitating. Accounts of
copious, disabling diarrhea are common, but
in most instances laboratory studies to rule out
a coinfection have not been performed.
Cryptosporidia has been known to infect
animals since 1907, but the first human infec-
tion was recognized in 1976. Although origi-
nally thought to infect only immunocompro-
mised individuals, since 1986 cryptosporidia
has been recognized as a worldwide parasite
that affects individuals of all ages. Organisms
are probably present in all major streams and
lakes in the United States.
Numerous therapeutic agents have been
tried for this organism; none have had any sig-
nificant benefit. The parasite is only elimi-
nated when the host develops immunity. As a
result, infestations are far worse in immuno-
compromised persons. Cryptosporidiosis is a
common first indication of the existence of
HIV infection, and a number of immunocom-
promised individuals die as the result of their
infestation. It is feared in the AIDS commu-
nity.
A number of bacteria cause diarrheal dis-
ease. Probably the most common is E. coli,
which produce endotoxins that are the most
frequent cause for "traveller's diarrhea."
Staphylocci produce toxins that cause gastro-
intestinal disease, and salmonella, shigella,
yersinia, and campylobacter colonize the in-
testinal tract and produce diarrheal disease.
Much "traveller's" diarrheal disease is the
result of viral infection, particularly by Nor-
walk-like agents (noroviruses) and rota-
viruses. These organisms can produce dehy-
drating diarrhea, although the infections rarely
last more than two or three days. Other water
borne viruses cause much more significant
infections. Hepatitis A has long been known
to result from fecal contamination of water.
Hepatitis E, which produces epidemics that
are associated with a 10 to 20 percent mortali-
ty during pregnancy, is predominantly trans-
mitted by contaminated water. Transfusion
and intravenous drug abuse typically transmit
hepatitis C, but a significant number of indi-
viduals with that infection have no history of
such incidents. Contaminated water has been
proposed as a source.
Other organisms such as coxsackie, polio,
and echoviruses enter the body through the
gastrointestinal tract, although their principle
manifestations are in other organs.
TECHNIQUES FOR WATER DISINFECTION
The currently available and reasonably
convenient methods for field water disinfec-
tion are heat, the combination of microfiltra-
tion and chemical treatment, and ultraviolet
