Viruses are nonliving particles that take over living cells in order to multiply. Viruses cause
many common diseases. For example, viruses cause colds and flu. Cold sores are caused
by the virusHerpes simplex. This virus is shown inFigure21.4. Antibiotics do not affect
viruses. However, medicines called antiviral drugs can treat many diseases caused by viruses.
Figure 21.4: TheHerpes simplexvirus, which is shown here, causes cold sores on the lips.
Viruses are extremely small particles. This one is greatly magnified. ( 1 )
How Pathogens Spread
Different pathogens spread in different ways. Some pathogens spread through food. They
cause food borne illnesses. These illnesses were discussed in theFood and Digestive System
chapter. Some pathogens spread through water. Giardia lamblia is one example. Water
can be boiled or purified in other ways to killGiardiaand most other pathogens. Several
pathogens spread through sexual contact. HIV is one example. It is a virus you will read
about below. Other pathogens that spread through sexual contact are discussed in the
Reproductive Systems and Life Stages chapter.
Many pathogens that cause respiratory diseases spread by droplets in the air. Droplets are
released when a person sneezes or coughs. Thousands of tiny droplets are released when a
person sneezes is shown inFigure21.5. Each droplet can contain thousands of pathogens.
Examples of pathogens spread in this way are the viruses that cause colds and flu. You may
get sick if you breathe in the pathogens.