Environmental Engineering Reference
In-Depth Information
Viruses can infect humans (it only takes a single virus particle to infect a
host) through recently contaminated drinking water, the relativity of time
being connected with the survival ability of the virus in natural and man-
made hostile environments.
At present, over 100 virus types are known to occur in human feces, and
an infected person may excrete as many as 10 6 infectious particles in 1 gram
of feces; thus, the potential for contamination is very great. Of those contact-
ing viruses, only a small percentage are infected, and of those infected only
about 2% may become recognizably ill. Assuming a 1% infection rate and a
2% illness rate, this means that 1 in every 5000 persons coming in contact
with a virus becomes ill, a very high rate if water is contaminated with fecal
matter (Tchobanoglous and Schroeder, 1987).
Isolation of viruses has improved considerably during the past 40 years.
They can be controlled by chlorination, but at much higher levels than are
necessary to kill bacteria. The viruses examined by the drinking water prac-
titioner are practically limited to enteric viruses (infections of the intesti-
nal tract). Some viruses that may be transmitted by water include infectious
hepatitis, adenovirus, polio, coxsackie, echoviruses, and Norwalk agent. A
virus that infects a bacterium is called a bacteriophage .
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Viruses cannot metabolize nutrients, produce and excrete wastes,
move around on their own, or even reproduce unless they are
inside another organism's cells. They are not even cells.
bacteriophage
Lewis Thomas (1974) observed that when humans “catch diphtheria it is
a virus infection, but not of us.” In other words, what humans have really
caught is a bacterium that is infected by the virus—humans simply “blun-
dered into someone else's accident” (p. 76). The toxin of diphtheria bacilli is
produced by organisms that have been infected with a bacteriophage (phage),
which is any viral organism whose host is a bacterium. Most bacteriophage
research has been carried out on the bacterium Escherichia coli , which is one
of the Gram-negative bacteria that water specialists are concerned with
because it is a typical coliform.
indicator Viruses
Considerable research has been accomplished in the last several decades
in an attempt to determine certain viruses as indicator viruses. In Drinking
Water and Health , the National Academy of Sciences (1977) reached the fol-
lowing conclusions:
 
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