Diseases

A disease is any abnormal condition that impairs the normal functioning of the body. Diseases are often divided into infectious diseases, caused by microorganisms that invade the body, and non-infectious diseases, which arise from genetic, environmental or lifestyle factors. Infectious diseases have shaped human history and remain a major focus of medicine and public health, because many can spread from person to person or from animals and the environment.

Infectious diseases are caused by several kinds of pathogen. Bacteria are responsible for illnesses such as cholera, diphtheria and leptospirosis, and many can be treated with antibiotics. Viruses cause conditions including hepatitis, influenza, herpes and the disease AIDS, which results from infection with HIV; viral infections do not respond to antibiotics and are managed in other ways. Parasites such as protozoa cause illnesses like giardiasis and cryptosporidiosis, while some serious diseases, including certain haemorrhagic fevers such as Ebola and Lassa fever, are caused by particularly dangerous viruses.

Pathogens spread through several routes, including contaminated water and food, the bite of insects and other vectors, direct contact, and airborne droplets. Public health relies on understanding these routes to interrupt transmission through clean water, sanitation, vaccination and the control of outbreaks. Accurate knowledge of how specific diseases arise and spread is essential for diagnosis, treatment and prevention, and for limiting the impact of epidemics.

Frequently asked questions

What is an infectious disease?

An infectious disease is an illness caused by a pathogen — such as a bacterium, virus, fungus or parasite — that can often be transmitted from one host to another.

What is the difference between HIV and AIDS?

HIV is the virus that infects and weakens the immune system. AIDS is the advanced stage of that infection, marked by severe immune damage and vulnerability to other illnesses.

How do infectious diseases spread?

They spread through routes such as contaminated water and food, airborne droplets, insect and animal vectors, and direct contact between people or with infected surfaces.