Page 5 - The Science of Infectious Diseases

noncommunicable
ber of human cells. “You’re more
microbe than you are human,” Not transmissible by direct contact
says Rob Knight, a professor of
pediatrics and computer science
and engineering at the University of California, San Diego. “The
current estimate is you’re about 43% human if you’re counting
up all the cells.” 4
Most of these microbes are harmless, and some are even ben-
efi cial. Some microbes in the digestive tract help break down foods
so nutrients can be absorbed into the body. Others stimulate the
immune system so it can fi ght disease.
And others prevent disease-causing or-
ganisms from colonizing the gut. “They communicable
are essential to your health,” says Ruth
Ley, director of the Department of Mi- Capable of being transmitted
crobiome Science at the Max Planck through direct contact
Institute for Developmental Biology in
Germany. “Your body isn’t just you.” 5
However, some microbes on the skin and in the environment
can cause sickness and even death if they fi nd a way inside the
body. These disease-causing microbes are known as pathogens.
There are four basic types of pathogen: bacteria, viruses, fungi,
and parasites. Each has developed unique ways of invading and
surviving within their human hosts. “Pathogenic microorganisms
are still here because they have found ways of avoiding elimina-
tion by their host or by the microbial competition,” writes Syed
Amin Tabish, editor in chief of the International Journal of Health
Sciences. “‘Successful’ pathogens have developed strategies to
enter the body and reach and choose their favourite niche, while
defying the powerful human immune systems.” 6

Bacteria
Bacteria are single-celled organisms that live in all kinds of
environments—in water, in soil, and on or inside plants and
animals. Bacteria have been found 26,247 feet (8,000 m) above

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