Pathogens are microorganisms that cause infectious disease. A pathogen may be a virus, a bacterium, a protist or a fungus. Pathogens can infect plants as well as animals. A disease caused by a pathogen is a communicable disease because it can be passed from one organism to another.
Pathogens are spread in three main ways: by direct contact, by water and by air. Touching an infected person is direct contact. Drinking water that contains pathogens spreads disease through water. Breathing in droplets from a cough or sneeze spreads disease through the air.
Bacteria and viruses may reproduce rapidly inside the body. Bacteria can make poisons called toxins, which damage tissues and make us feel ill. Viruses live and reproduce inside cells, and this causes cell damage.
The spread of disease can be reduced or prevented by cutting the route of infection. Washing hands, preparing food hygienically, keeping drinking water clean, vaccination and keeping infected people away from others all help.