Page 4 - The Science of Infectious Diseases

CHAPTER ONE

Menacing Microbes

Human beings suffer from two basic kinds of disease: in-
fectious diseases and noninfectious, or noncommunicable,
diseases (NCDs). NCDs originate inside the body and can-
not be spread to other people. Heart disease, cancer, and
diabetes are three of the best-known NCDs. NCDs are re-
sponsible for most deaths worldwide, claiming the lives of
41 million people per year. Seven out of every ten deaths are
caused by NCDs, according to the WHO.
The other main killers are infectious diseases, or commu-
nicable diseases. About two out of every ten deaths world-
wide are caused by infectious diseases. (The other 10 percent
of deaths are from accidents, drug overdoses, and suicides.)
Infectious diseases do not originate inside the body. Instead,
they develop when microorganisms—organisms so small
they can only be seen with a microscope—invade the body
from the outside and disrupt its healthy functioning. Some
infectious diseases, including measles, mumps, and chicken
pox, make a person sick but usually are not life threatening.
Other infectious diseases, including COVID-19, HIV/AIDS,
and tuberculosis, can kill, and they often do.

Helpful Microbes
Not all microorganisms, or microbes, make a person sick.
Countless microbes live on a person’s skin, in the mucous
membranes, and deep in the digestive tract. Ten trillion
microbes reside in the gut alone. In fact, the number of
microbial cells in the human body is greater than the num-

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