An epidemic is a sudden disease outbreak that affects a large number of people—larger than what is normally expected—in a particular region, community, or population. A pandemic occurs when an epidemic spreads to a larger region, or across multiple nations or continents.

An epidemic, and later pandemic, of an infectious disease can happen if the virus, bacteria, or other cause of the disease has recently grown stronger, is introduced somewhere it has never been before, is newly transmitted to a place where more people are exposed to it, or finds new ways to enter the bodies of those it is affecting. It also can happen if people somehow grow more susceptible to the cause of the disease or have greater exposure to it. Pandemics often are caused by viruses that start in animals and later infect humans.

In the early 1800s, a cholera epidemic turned into a pandemic. The epidemic began in Bengal, India and grew into a pandemic as it travelled across India and over to China and other Asian countries. An influenza epidemic turned into a deadly pandemic in 1918, when it spread around the globe. The close quarters and moving troops of World War I helped fuel its expansion. Of the five hundred million who were infected, at least fifty million died.

One of the worst pandemics in recorded history was the plague, or Black Death, in the fourteenth century Europe was hardest hit, with as much as half of the continent’s population, or about twenty-five million, dying of the disease within four years. It was known as the Black Death because of the black skin spots that accompanied it.

HIV/AIDS is an ongoing pandemic. Human Immunodeficiency Virus is the agent that causes Acquired Immune Deficiency Syndrome, which is the most advanced stage of an HIV infection. HIV attacks and weakens a person’s immune system. AIDS was first recognized as a new disease in 1981, as young homosexual men died as a result of unusual infections and cancers. HIV/AIDS mainly spreads through sexual contact, but can be transmitted through body fluids like blood, semen, vaginal fluids, and breast milk.

Since the pandemic began, between 59.9 and 100 million people have been infected with HIV, and between 25 and 49.9 million people have died from AIDS-related diseases. As of 2017, between 31.1 and 43.9 million people living around the world were infected with HIV. While medical advances have helped to better treat HIV and reduce the number of AIDS-related deaths, not all countries have access to these treatments. Developing countries have the highest rates of infection and AIDS-related deaths, with the highest rates in sub-Saharan Africa.