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11.53: Pathogens

Difficulty Level: At Grade Created by: CK-12
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What are germs?

You know germs can make you sick. They can live anywhere, from on your doorknob to in your food. A more scientific word for germ is pathogen.


Has this ever happened to you? A student sitting next to you in class has a cold. The other student is coughing and sneezing, but you feel fine. Two days later, you come down with a cold, too. Diseases like colds are contagious. Contagious diseases are also called infectious diseases. An infectious disease is a disease that spreads from person to person.

Infectious diseases are caused by pathogens. A pathogen is a living thing or virus that causes disease. Pathogens are commonly called “germs.” They can travel from one person to another.

Types of Pathogens

Living things that cause human diseases include bacteria, fungi, and protozoa. Most infectious diseases caused by these organisms can be cured with medicines. For example, medicines called antibiotics can cure most diseases caused by bacteria. Bacteria are one-celled organisms without a nucleus. Although most bacteria are harmless, 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 in Figure below. Some types of pneumonia and many cases of illnesses from food are also caused by bacteria.

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.

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 Figure below. A more serious fungus disease is histoplasmosis. It is a lung infection.

Ringworm isn’t a worm at all. It’s a disease caused by a fungus. The fungus causes a ring-shaped rash on the skin, like the one shown here.

Protozoa are one-celled organisms with a nucleus. They cause diseases such as malaria. Malaria is a serious disease that is common in warm climates. The protozoa infect people when they are bit by a mosquito. More than a million people die of malaria each year. Other protozoa cause diarrhea. An example is Giardia lamblia, which is shown in Figure below.

This picture shows a one-celled organism called Giardia lamblia. It is a protozoan that causes diarrhea.

Viruses are nonliving collections of protein and DNA that must reproduce inside of living cells. Viruses cause many common diseases. For example, viruses cause colds and the flu. Cold sores are caused by the virus Herpes simplex. This virus is shown in Figure below. Antibiotics do not affect viruses, because antibiotics only kill bacteria. But medicines called antiviral drugs can treat many diseases caused by viruses.

The Herpes simplex virus, which is shown here, causes cold sores on the lips. Viruses are extremely small particles. This illustration is greatly magnified.

How Pathogens Spread

Different pathogens spread in different ways. Some pathogens spread through food. They cause food borne illnesses, which are discussed in a previous lesson. Some pathogens spread through water. Giardia lamblia is one example. Water can be boiled to kill Giardia and most other pathogens.

Several pathogens spread through sexual contact. HIV is one example, which is discussed in the next lesson. Other pathogens that spread through sexual contact are discussed in a separate lesson.

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, as shown in Figure below. Each droplet can contain thousands of pathogens. Viruses that cause colds and the flu can spread in this way. You may get sick if you breathe in the pathogens.

As this picture shows, thousands of tiny droplets are released into the air when a person sneezes. Each droplet may carry thousands of pathogens. You can’t normally see the droplets from a sneeze because they are so small. However, you can breathe them in, along with any pathogens they carry. This is how many diseases of the respiratory system are spread.

Pathogens on Surfaces

Other pathogens spread when they get on objects or surfaces. A fungus may spread in this way. For example, you can pick up the fungus that causes athlete’s foot by wearing shoes that an infected person has worn. You can also pick up this fungus from the floor of a public shower. After acne, athlete’s foot is the most common skin disease in the United States. Therefore, the chance of coming in contact with the fungus in one of these ways is fairly high.

Bacteria that cause the skin disease impetigo can spread when people share towels or clothes. The bacteria can also spread through direct skin contact in sports like wrestling.

Pathogens and Vectors

Still other pathogens are spread by vectors. A vector is an organism that carries pathogens from one person or animal to another. Most vectors are insects, such as ticks and mosquitoes. When an insect bites an infected person or animal, it picks up the pathogen. Then the pathogen travels to the next person or animal it bites. Ticks carry the bacteria that cause Lyme disease. Mosquitoes, like the one in Figure below, carry West Nile virus. Both pathogens cause fever, headache, and tiredness. If the diseases are not treated, more serious symptoms may develop.

Some diseases are spread by insects. The type of mosquito shown here can spread West Nile virus. The virus doesn’t make the mosquito sick. The mosquito just carries the virus from one person or animal to another.

The first case of West Nile virus in North America occurred in 1999. Within just a few years, the virus had spread throughout most of the United States. Birds as well as humans can be infected with the virus. Birds often fly long distances. This is one reason why West Nile virus spread so quickly.


  • infectious disease: A disease that spreads from person to person.
  • pathogen: A living thing or virus that causes disease.
  • vector: An organism that carries pathogens from one person or animal to another.


  • Infectious diseases are caused by bacteria, fungi, protozoa, or viruses that can travel from one person to another.
  • Infectious diseases can be spread by a vector, an organism that carries pathogens from one person or animal to another.


Use the resource below to answer the questions that follow.

  1. What is Yersinia pestis? What disease does it cause?
  2. What did Louis Pasteur discover about microorganisms? How was this discovery related to disease?
  3. What do pathogens have in common?
  4. What are ectoparasites? Where do they live? How do they differ from endoparasites?
  5. What group of fungi cause many diseases?


  1. What is a pathogen?
  2. Explain why using insect repellent reduces your risk of developing Lyme disease.

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infectious disease Disease that spreads from person to person.
pathogen Living thing or virus that causes disease.
vector Organism that carries pathogens from one person or animal to another.

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Difficulty Level:
At Grade

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7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Dec 02, 2016
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