another. This is why the diseases they cause are “catching.”
Types of Pathogens
Living things that cause human diseases include bacteria, fungi, and protozoa. Most in-
fectious diseases caused by these organisms can be cured with medicines. For example,
medicines called antibiotics can cure most diseases caused by bacteria.
Bacteriaareone-celledlivingthingswithoutanucleus. Althoughmostbacteriaareharmless,
some cause diseases. Worldwide, the most common disease caused by bacteria is tuberculosis
(TB). TB is a serious disease of the lungs. Another common disease caused by bacteria is
strep throat. You may have had strep throat yourself. Bacteria that cause strep throat are
shown inFigure21.1. Some types of pneumonia and many cases of food borne illnesses are
also caused by bacteria.
Figure 21.1: The structures that look like strings of beads are bacteria. They belong to
the genus Streptococcus. Bacteria of this genus cause diseases such as strep throat and
pneumonia. They are shown here 900 times bigger than their actual size. ( 4 )
Fungi are simple organisms that consist of one or more cells. They include mushrooms and
yeasts. Human diseases caused by fungi include ringworm and athlete’s foot. Both are skin
diseases that are not usually serious. What a ringworm infection looks like is shown in
Figure21.2. A more serious fungus disease is histoplasmosis. It is a lung infection.
Protozoa are one-celled eukaryotes (with a nucleus). They cause diseases such as malaria.
Malaria is a serious disease that is common in warm climates; the protozoa is transferred to
people by a mosquito. More than a million people die of malaria each year. Other protozoa
cause diarrhea. An example isGiardia lamblia, which is shown inFigure21.3.