Pathogens and transmissible disease

A pathogen is an organism or infectious agent that causes disease. When a pathogen enters a host—a living organism in which it lives—it can reproduce or act in ways that damage the host’s body. This damage can lead to illness.

A transmissible disease is a disease whose pathogen can pass from one host to another. If an infected host passes the pathogen to another host, the pathogen can cause an infection there too. Further transmission can continue the chain of infection, allowing the disease to spread through a population.

Not all diseases are caused by pathogens. For example, a disease caused by an inherited condition is not transmissible in this sense: no pathogen passes between hosts. The distinction is between the pathogen, which is the cause, and the disease, which is the illness it can cause.

Hygiene and public health control of disease

Disease spreads when pathogens—microorganisms that cause disease—pass from a source of infection to people. Hygiene and public health measures reduce spread by blocking these transmission pathways or removing sources of infection.

Clean water, sewage and waste

A clean water supply prevents people from drinking waterborne pathogens or using contaminated water in the home. Treating sewage reduces the release of human waste containing pathogens into the environment, where it could contaminate water or other surroundings.

Safe waste disposal removes rubbish and other materials that could harbour pathogens. It also reduces the attraction of animals that can carry and spread disease.

Food and personal hygiene

Hygienic food preparation reduces contamination of food and helps prevent pathogens from multiplying in it. This lowers the chance that someone will swallow pathogens when they eat.

Good personal hygiene, including washing hands and keeping the body clean, reduces the transfer of pathogens between people, surfaces and food. For example, handwashing can remove pathogens before they are transferred from a person's hands to food.

These measures protect whole communities as well as individuals: when fewer pathogens reach people, there are fewer opportunities for infection to spread to others.

Vaccination

Vaccination prepares the immune system to respond to a pathogen before the body encounters it in the future. A vaccine introduces weakened pathogens or their antigens into the body. Antigens are molecules that the immune system can recognise as foreign.

The antigens stimulate white blood cells called lymphocytes. Some lymphocytes produce antibodies that specifically bind to the antigens. Others become memory cells, which remain in the body after the initial response.

Protection after vaccination

If the same pathogen enters the body later, memory cells recognise its antigens and allow a faster, stronger antibody response. The pathogen can then be controlled before it causes illness. Because memory cells remain in the body, they can provide long-term immunity.

Vaccination and disease spread

When vaccination prevents people from becoming infected, the pathogen has fewer opportunities to pass from one host to another. Vaccinating people can therefore help control the spread of disease as well as protect individuals.

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.

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.