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Ports and disease

Infectious disease

What is an infectious disease?

The Variola virus.
View full size imageThe 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


Small pox graphic
View full size image

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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