How many diseases are classed as pandemics?

Answer 1

First, it's important to understand the difference between a pandemic and an epidemic. An epidemic is a diseases where infection rates are significantly higher than what would normally be expected from that disease in that population.

A pandemic is a epidemic that has been spread across a large region, usually multiple continents. Some examples are HIV/AIDS, influenza, smallpox and malaria. To be classified as a true pandemic, the disease must not only kill many people, and be spread over a large area, it must also be infectious. Cancer is a good example of this: even though it kills lots of people, and is very widespread, it is not pandemic, because most are not contagious.

Lots of things can contribute to something becoming a pandemic. Diseases that are resistance to antibiotics or antivirals are considered risks of growing into pandemics. Highly contagious diseases, which have multiple ways of infecting someone would be more likely candidates, as are diseases which can move between species. An example is the Bubonic Plague, also known as the Black Death, which is thought to have killed up to a third of the population across Europe in the 14th Century. It was a very large pandemic, which was carried by fleas. The fleas would become infected, and bite humans, who would then get the disease. These infected fleas would hitch rides with rats, which lived close around humans or would climb on to ships bound for far away places. The rats would then leave the ship, the fleas would jump off and infect people in this new place, and the disease spread. The people were also contagious, so it spread between people, as well as being carried by the fleas.