Infection happens when microorganisms (like bacteria, viruses, fungi or parasites) get inside the body and multiply. Most of the time, germs are harmless or even helpful, but under certain conditions they can make us sick.

You can get some infections by direct contact with another person, usually through the mouth, nose or genitals. Bacterial diseases like pertussis (whooping cough), tuberculosis and strep throat spread this way. You can also get some bacterial infections through the air from droplets of water or mucus (like phlegm or snot) or by touching contaminated surfaces. Some bacterial infections, such as Rocky Mountain spotted fever, Lyme disease and chlamydia, are spread by bugs (like mosquitoes, ticks or fleas) that carry the germs.

Some infections come from fungi, which live in the soil and on plants. Fungi that cause diseases in people include tinea corporis, tinea pedis and candida. Others, such as the parasites that cause scabies and head lice, live on the skin.

Parasites are single-celled organisms that either live off or in people, including worms and tapeworms. Many parasites are benign, but some can be deadly.

Infections can be prevented by washing hands often, covering coughs and sneezes, not sharing personal items such as toothbrushes or combs and using alcohol-based hand sanitizers. In hospitals, infection rates are lowered by practicing standard precautions and implementing effective hand hygiene programs. This includes putting on new gloves before every patient interaction, avoiding unnecessary procedures and identifying patients with infections that require immediate treatment.