Pathogens are microorganisms that cause infectious disease. A pathogen may be a virus, a bacterium, a protist or a fungus, and it may infect animals or plants. Diseases caused by pathogens are called communicable diseases because they can pass from one organism to another.
Pathogens can be spread by direct contact, by water or by air. Touching an infected person is direct contact, drinking contaminated water spreads pathogens in water, and breathing in droplets from a cough spreads them through the air.
Once inside the body, bacteria and viruses can reproduce rapidly. Bacteria may produce poisons called toxins that damage tissues and make us feel ill. Viruses live and reproduce inside cells, causing cell damage.
The spread of disease can be reduced or prevented by breaking the route of infection. Examples are washing hands, preparing food hygienically, keeping drinking water clean, vaccination, and keeping infected people away from others.