Communicable disease

A communicable disease is an infection that can spread from an infected person or animal to others. It is caused by a pathogen, such as a bacterium, virus, fungus, protozoan or parasitic worm, or by a toxin the pathogen makes. Spread can be direct, through touch, sexual contact, droplets from coughs and sneezes, or from mother to baby, or indirect, through the air, contaminated food, water or objects, or a vector such as a mosquito.

Textbooks draw useful lines between related words. Every communicable disease is infectious, but not every infectious disease is communicable: tetanus comes from bacterial spores in soil entering a wound and does not pass between people. Contagious diseases, such as measles, are communicable diseases that spread especially easily. Some health agencies use a broader definition that also counts infections picked up from animals, vectors or the environment.

Communicable diseases are controlled by breaking the chain of infection, through vaccination, clean water and sanitation, hand hygiene, safe food, condoms, insect control, isolation and quarantine, and treatment that shortens the time a person is infectious. In global health statistics they are often grouped with maternal, newborn and nutritional conditions, and they remain a leading cause of death in low-income countries, where tuberculosis, HIV and malaria take a heavy toll.