An epidemic is when a disease spreads quickly through a population, infecting a large percentage. Disease is spread by infection. When a person with a disease passes it to a new person, they 'infect' that new person. Some diseases travel from one person to the next very easily and the epidemic is very fast. For example, in 1919, Spainish Flu raced around the world killing maybe 20 million people in a couple of years. Spainish flu was very very 'contagious' and could jump from one person to the next very quickly. Sneezing projects the flu virus into the air and then it's breathed in by another person who becomes infected. This is an air-borne disease.
Other epidemics are slower. The AIDS virus cannot travel across the air in sneezes, AIDS is passed when an infected person has sex with another person. This is a sexually transmitted disease. So the AIDS epidemic travels more slowly.
Infected insects often trasmit disease by biting people and giving them the disease.
Dirty food and dirty water often spread disease. The largest cause of disease on the planet is probably dirty water. The World Health Organisation estimates 25 million people a year die because of infections from water-borne diseases.
In the end, disease is due to lack of hygiene. Clean water, clean food, clean hands and isolating infected people reduces the chance of epidemics.
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