What is an infectious disease?
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 The Variola virus. © Centers for Disease Control | To understand how diseases have travelled by sea over the centuries, it is necessary to understand what causes infectious diseases, and how disease can be carried from one place to another.
A disease is infectious if it can be passed from one person to another. All infectious diseases are caused by an agent – mainly microbes such as bacteria and viruses.
How diseases spread
All diseases have one or more ways of spreading – or modes of transmission.
| Mode of transmission |
Method of transmission |
Typical diseases |
| Airborne |
droplets from the nose or mouth |
smallpox, SARS, pneumonic plague, influenza |
| Waterborne and foodborne |
consumption of contaminated food or drink |
cholera, hepatitis A, typhoid fever |
| Vector-borne |
bites from insects or ticks |
malaria, yellow fever |
| Bloodborne |
contact with blood |
HIV/AIDS, hepatitis B and C, malaria |
| Zoonoses |
contact with animals |
rabies, brucellosis |
| Sexually transmitted diseases |
unsafe sexual contact |
HIV/AIDS, hepatitis B, syphillis |
| Soil-transmitted |
contact with contaminated soil |
anthrax, tetanus |
Epidemics
Some diseases cannot be transmitted easily. A good example is rabies - a deadly disease but one that only people bitten by infected animals can catch.
Where transmission depends on such very specific conditions, relatively small numbers of people are affected at any one time.
However, some diseases can be transmitted far more easily. Airborne and waterborne diseases spread particularly rapidly in the right conditions.
As this example shows, one infected person can pass on a disease to many other people. Epidemics are serious outbreaks of a disease affecting large numbers of people at the same time.
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