Disease transmission

Communicable diseases can pass between hosts. In animals and plants, these diseases may be caused by pathogens: viruses, bacteria, protists or fungi. A host is an organism that a pathogen can infect.

Transmission occurs when a pathogen passes from an infected host or a contaminated source to another host. If the pathogen enters and establishes itself in the new host, it can cause an infection. The route by which it travels depends on the pathogen and the host.

Routes of disease transmission

A pathogen can reach a new host by several routes:

• Direct contact: the pathogen transfers when hosts touch.
• Water: water carrying the pathogen reaches a new host.
• Air: droplets or particles carrying the pathogen reach animal or plant tissues.

Breaking a transmission route

A pathogen has fewer opportunities to cause infection if its route to a susceptible host is interrupted. For example, isolating an infected host reduces its contact with other hosts. Removing infected plants can likewise limit contact between a pathogen and plants that could be infected.

Bacterial pathogens

Some bacteria can enter a host and cause disease. These pathogenic bacteria are microorganisms made of a single cell. Once inside a host, they can multiply rapidly by cell division, increasing the number of bacteria present.

Some pathogenic bacteria also produce toxins: poisonous substances that damage the host’s tissues. This damage contributes to the symptoms of illness.

Bacterial populations can therefore harm a host through their multiplication and, when the bacteria produce toxins, through the effects of those toxins.

Prevention of infectious disease spread

Infectious diseases spread when pathogens pass from a source to a host they can infect. Spread can be reduced by limiting contact with pathogens or by reducing the number of susceptible hosts: those that can be infected.

Limiting contact with pathogens

Hygiene measures remove or kill pathogens before they can pass between hosts. Handwashing removes pathogens from hands, while cleaning or disinfecting shared surfaces and tools reduces pathogens that could be transferred between animal or plant hosts.

Treating water reduces the pathogens it can carry, making infection through contact with or use of the water less likely. Ventilation brings fresh air into a space, reducing the accumulation of infectious airborne particles that others might breathe in.

Vaccination and spread

Vaccination stimulates an immune response in an animal host before it encounters the pathogen. This can protect the host against infection.

Where vaccination reduces infection or transmission, the pathogen has fewer opportunities to pass between hosts. Vaccination can therefore reduce spread as well as protect vaccinated individuals.

No single preventive measure necessarily gives complete protection. Using measures together can reduce the opportunities for pathogens to reach susceptible hosts.

Viral pathogens

Viruses are infectious particles that reproduce inside living host cells. A virus enters a host cell and uses the cell’s machinery to make its own components. These components are assembled into new viruses, which can be produced rapidly.

The new viruses leave the infected cell and can infect other cells. Viral reproduction and release can damage or destroy the host cell. As more cells are affected, this damage can contribute to illness.