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Infectious disease

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An infectious disease is a sickness that happens when an organism (a living thing such as a plant or animal) is attacked by a pathogen. Pathogens (such as bacteria, viruses, and other germs) are too small to see. Some infectious diseases are contagious, which means that the sick plant, animal, or person can get other things sick. The pathogen can get from one organism to another through air, food, water, blood, or physical touch.

Usually, the sick organism heals. Then, the organism's body learns how to fight the pathogen. If the pathogen attacks again, the immune system knows how to fight it. Thanks to the immune system, people can not get the same disease twice. When an organism's body knows how to fight a disease, the body is immune to that pathogen.

Here are some types of pathogens:

Infectious disease is a large cause of pain and death. The World Health Organization says that infectious diseases cause about 25% of all deaths. These three pathogens cause the most deaths: HIV (the virus that causes AIDS), Tuberculosis, and Malaria.