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.

Transmission of pathogens

A pathogen can spread when it is transferred from an infected host—a person or other organism carrying it—to another host. This transfer is called transmission. It can happen through direct contact or by an indirect route.

Direct contact

A pathogen can pass directly from one host to another when they touch. It can also be transmitted through the exchange of blood or other body fluids. For example, contact with infected blood can transfer a pathogen to another person.

Indirect transmission

In indirect transmission, something carries the pathogen between hosts. The main routes include:

• Contaminated surfaces: a person touches a surface carrying a pathogen and then transfers it to their body.
• Contaminated food: a person eats food containing a pathogen.
• Animals: an animal carries a pathogen and transfers it to another host, for example through a bite.
• Air: infectious droplets or particles enter the air and are breathed in by another person.

Transmission routes and prevention

Preventive actions depend on how a pathogen spreads. Cleaning surfaces and washing hands can reduce transmission by contact with contaminated surfaces. Safe food handling can reduce transmission through food, while protection from bites can reduce transmission by animals. Ventilation can reduce the concentration of infectious particles in the air. Avoiding contact with another person’s blood or other body fluids can reduce transmission by those routes.

Body defences against pathogens

Pathogens are microorganisms that can cause disease. The body has defences that block or trap pathogens before they enter, and others that deal with pathogens that get past these barriers.

Barriers to pathogen entry

The skin forms a physical barrier that blocks many pathogens from entering the body.

In the breathing system, hairs in the nose and mucus help trap particles and microorganisms from the air before they reach deeper parts of the system.

Pathogens can also be swallowed in food or water. Stomach acid kills many of them before they can cause disease.

White blood cells

Some pathogens get past the body’s first barriers. White blood cells help defend the body by attacking and destroying these pathogens.

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.

Antigen–antibody recognition and action

Pathogens have antigens: molecules with specific shapes that can be recognised by the immune system. An antibody is a protein with binding sites whose shapes are complementary to a particular antigen. The matching shapes allow the antibody to bind to that antigen.

Because antibody binding sites are specific, an antibody binds to its matching antigen rather than to every pathogen. Pathogens with different antigens therefore require antibodies with different complementary binding sites.

Binding can lead directly to the pathogen’s destruction. It can also mark the pathogen for destruction by phagocytes, which are cells that engulf and digest pathogens.

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.

Passive immunity

Passive immunity is short-term protection provided by antibodies acquired from another individual. The recipient does not make these antibodies through their own immune response; the antibodies are already available to help defend against infection.

A mother can pass antibodies to her infant across the placenta before birth and through breast milk after birth. Breast-feeding therefore helps protect the infant by supplying maternal antibodies.

This protection is temporary. The acquired antibodies are gradually broken down, and passive immunity does not produce memory cells in the recipient to support a lasting immune response.

Cholera

Cholera is a disease caused by a bacterium transmitted through contaminated water. After entering the gut, the bacterium produces a toxin that causes chloride ions to be secreted into the small intestine.

The chloride ions make the contents of the small intestine more concentrated. Water then moves into the gut by osmosis: it crosses a partially permeable membrane towards the more concentrated contents of the intestine. The extra water causes diarrhoea.

Diarrhoea removes water and ions from the body. Their loss from the blood leads to dehydration and an imbalance of ions.