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What are infectious diseases?

Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi. These can be spread directly or indirectly from one person to another or else from animals, latter known as zoonotic diseases.

 For an organism to cause infection, it needs to follow a chain of infection. An infectious disease result from the interaction of an agent (organism), host (reservoir) and the environment. Transmission of infection occurs when the agent leaves the reservoir through a portal of exit and enters through an appropriate portal of entry to infect a susceptible host (as seen below).

 Transmission can be by direct contact e.g. droplet spread or else indirect contact such as airborne transmission, vehicle-borne or vector borne transmission.