Environmental Engineering Reference
In-Depth Information
95% of the new cases occur. A second problem is that
most strains of the TB bacterium have developed ge-
netic resistance to almost all effective antibiotics.
Population growth and urbanization have also led
to increased contacts between people and spread TB,
especially in areas where large numbers of the poor
crowd together. In addition, the spread of AIDS is a
factor because the disease greatly weakens the im-
mune system and allows TB bacteria to multiply in
people who have AIDS.
Slowing the spread of the disease requires early
identification and treatment of people with active TB,
especially those with a chronic cough. Treatment with a
combination of four inexpensive drugs can cure 90% of
individuals with active TB. To be effective, the drugs
must be taken every day for 6-8 months. Because the
symptoms disappear after a few weeks, many patients
think they are cured and stop taking the drugs. This al-
lows the disease to recur in a drug-resistant form. It
then spreads to other people, and drug-resistant strains
of TB bacteria develop.
Critical thinking: What three
things would you do to reduce the spread of TB?
birds that carry the virus. It can infect 230 species of
animals, including as least 130 bird species. In the
United States, it was first reported in 1999, when it was
apparently introduced by a mosquito or bird from the
Middle East. Since then, the virus has spread through-
out most of the lower 48 U.S. states, infecting thou-
sands, and killing several hundred people. The chance
of being infected and killed by this disease is low
(about 1 in 2,500). In 2004, the flu killed more Ameri-
cans in two days than the West Nile virus killed during
the entire year.
The
severe acute respiratory syndrome
(
SARS) virus
was first transmitted to humans in China in 2002 from
the flesh of wild animals valued as delicacies. From
these wild hosts, this highly infectious disease it was
transmitted to pigs, chickens, and ducks eaten by hu-
mans. It spread to chefs and animal handlers and then
to other humans in close contact with SARS-infected
people.
Its flu-like symptoms include chills, fever, a dry
cough, headaches, and muscle pains; in the elderly
and sick, SARS can quickly turn into life-threatening
pneumonia. In 2003, the disease began spreading to
other countries, mostly by global travelers. Within six
months it had reached 31 other countries, infecting at
least 8,500 people and causing 812 deaths. Swift local
action by the WHO and other health agencies helped
contain the spread of this disease by July 2003. With-
out careful vigilance, it might break out again.
Health officials are concerned about the emer-
gence and spread of these three and other
emerging
viral diseases
and are working hard to control their
spread. Nevertheless, in terms of annual infection
rates and deaths, the three most dangerous viruses by
far remain HIV, flu, and HBV.
You can greatly reduce your chances of getting in-
fectious diseases that spread from person to person by
practicing good old-fashioned hygiene. Wash your
hands thoroughly and frequently, and avoid touching
your mouth, nose, and eyes.
Science: Viral Diseases
HIV, flu, and hepatitis B viruses infect and kill many
more people each year than the highly publicized
Ebola, West Nile, and SARS viruses.
What are the world's three most widespread and dan-
gerous viruses? The biggest killer is the
human immun-
odeficiency virus (HIV),
which is transmitted by unsafe
sex, sharing of needles by drug users, infected mothers
who pass the virus to their offspring before or during
birth, and exposure to infected blood. On a global
scale, HIV infects at least 5 million new people each
year (about 41,000 in the United States), half of them
women. The resulting complications from AIDS kill
about 3 million people annually (Case Study, right).
The second biggest killer is the
influenza
or
flu
virus, which is transmitted by the body fluids or air-
borne emissions of an infected person, and kills about
1 million people per year. In 1918, an especially dan-
gerous flu virus spread rapidly around the globe and
killed an estimated 30 million people.
The third largest killer is the
hepatitis B virus
(HBV),
which damages the liver and kills about 1 mil-
lion people each year. Like HIV, it is transmitted by un-
safe sex, sharing of needles by drug users, infected
mothers who pass the virus to their offspring before or
during birth, and exposure to infected blood.
In recent years, three other viruses that cause pre-
viously unknown diseases have received widespread
coverage in the media. The
Ebola virus
is transmitted
by the blood or other body fluids of an infected person.
The
West Nile virus
is transmitted by the bite of a
common mosquito that became infected by feeding on
Science Case Study: HIV and AIDS
The spread of AIDS, caused by infection with HIV, is
one of the world's most serious and rapidly growing
health threats.
The global spread of
acquired immune deficiency syn-
drome (AIDS),
caused by infection with HIV, is consid-
ered the world's most serious and rapidly growing
health threat. The virus itself is not deadly, but it kills
immune cells and leaves the body defenseless against
infectious bacteria and other viruses.
According to the WHO, by the beginning of 2004
some 39.4 million people worldwide (96% of them
in developing countries, especially African countries
south of the Sahara Desert) were infected with HIV.
