Health and Communicable Diseases

A communicable disease can be passed between individuals because its pathogen can spread from one host to another. A host is an organism in which a pathogen lives. Transmission does not always require the hosts to touch: pathogens can travel through water, air or an animal that carries them.

A non-communicable disease cannot be passed between individuals. Examples include cardiovascular disease, asthma and diabetes. Genetic and lifestyle factors can influence the risk of developing some of these conditions, but contact with an affected person does not transmit the disease.

The distinction is about transmission, not severity or duration. Both types can seriously affect health.

Pathogens cause disease in animals and plants

A pathogen is a disease-causing organism. The four pathogen groups you need to recognise are viruses, bacteria, fungi and protists. Not all microorganisms cause disease; the word pathogen applies to those that do.

Viruses enter host cells and use the cells' machinery to make copies of themselves. Damage to infected cells can cause illness. HIV is a viral pathogen.

Bacteria cause infections such as cholera, tuberculosis and chlamydia. Fungi can also cause disease: the fungus responsible for Chalara ash dieback infects ash trees. Malaria is caused by a parasitic protist, which lives in its host and damages the blood and liver.

Knowing the pathogen group is only part of understanding an infection. You also need to connect what it damages, how it spreads and how that route can be interrupted.

Cholera: interrupting waterborne transmission

Cholera is caused by bacteria that infect the intestines and cause diarrhoea. Severe diarrhoea causes substantial water loss and can become life-threatening.

Cholera spreads through contaminated water. When water becomes contaminated with faeces containing the bacteria, someone who drinks it may become infected. This connects poor sanitation with the spread of cholera.

Providing clean drinking water and keeping sewage away from water supplies interrupt this route. Good hygiene, including hand washing, also helps prevent faecal microorganisms reaching food or the mouth. The important connection is that fewer people swallow the bacteria, so fewer new infections occur.

Tuberculosis: reducing airborne transmission

Tuberculosis (TB) is caused by bacteria and causes lung damage. Symptoms can include a persistent cough and coughing up bloody mucus.

TB spreads through the air. An infected person can release bacteria in droplets when coughing; another person may breathe them in. This is a form of droplet infection.

Reducing close contact with infected people and avoiding crowded conditions reduce opportunities for transmission. Good ventilation helps remove pathogen-containing air from enclosed spaces. Covering coughs with a tissue helps limit the release of infectious droplets. These measures target the route from an infected person's lungs to another person's lungs.

Chalara ash dieback: airborne spread in plants

Chalara ash dieback is caused by a fungus. It causes leaf loss and bark lesions in ash trees. A lesion is an area of damaged tissue; affected leaves may also develop dark patches.

The fungus spreads through airborne spores, which can travel long distances in the wind. Airborne transmission therefore includes more than droplets from coughing: fungal spores can carry infection between plants.

Restricting imports of potentially infected trees helps prevent the disease being introduced to new areas. Removing infected trees can reduce sources of infection, and replacing them with unaffected species reduces the number of susceptible trees at that site. Because spores can arrive on the wind, local action cannot guarantee that all further transmission will stop.

Malaria: stopping an animal vector

Malaria is caused by a protist and causes damage to the blood and liver. Infected people may experience fever, chills and fatigue, and severe malaria can be fatal.

Mosquitoes carry the malaria pathogen between people. A mosquito acts as an animal vector: it transfers the pathogen through its bites rather than being the pathogen itself.

Mosquito nets and insect repellents reduce bites, making it less likely that the protist will reach another person. Insecticides can reduce mosquito numbers. Removing mosquito breeding habitats helps prevent the vector reproducing. These methods interrupt transmission by reducing contact with the vector or reducing the vector population.

Chlamydia and HIV: preventing sexual transmission

A sexually transmitted infection (STI) is an infection that can spread through sexual contact. Chlamydia is caused by bacteria, whereas HIV is caused by a virus. Both can spread during sexual contact involving infected bodily fluids, such as semen and vaginal fluid.

Chlamydia often causes no symptoms. When symptoms occur, they can include painful urination, pelvic pain or unusual discharge. Untreated infection can lead to infertility. Having no symptoms does not mean that an infected person cannot pass it on.

Condoms provide a physical barrier that reduces contact with infected fluids and therefore reduces transmission. Abstaining from sexual activity prevents transmission through that route. Screening can identify infections, including symptomless cases, so affected people can receive treatment and take steps to avoid passing the infection on.

HIV can also spread through infected blood. Avoiding shared needles therefore reduces another route of transmission. Preventing sexual transmission alone does not prevent every possible route by which HIV can spread.

Why one disease can increase susceptibility to another

Susceptibility means how likely someone is to develop a disease. One disease can increase susceptibility to others if it weakens the body's defences or otherwise affects health.

HIV provides a clear biological example. It infects and destroys white blood cells involved in the immune system, including a type of lymphocyte. With fewer functioning immune cells, the body becomes less able to fight other pathogens. Infections that would normally be controlled can then cause serious illness.

The connection is:

HIV infection → destruction of white blood cells → weakened immune defence → increased susceptibility to other infections.

If untreated, HIV can lead to AIDS, when severe immune damage allows life-threatening illnesses to develop. HIV and AIDS are therefore not interchangeable names: HIV is the virus, while AIDS is a condition that can result from its effects.

Diseases can also interact in other ways. For example, infection with certain types of human papillomavirus (HPV) increases the risk of cervical cancer. The infection is communicable, but the resulting cancer is not passed between people. Increased risk does not mean that everyone with the first disease will develop the second.

Break the transmission route

• Cholera: clean drinking water, sanitation and hand washing.
• TB: reduce contact with infected people, improve ventilation and cover coughs.
• Ash dieback: restrict movement of infected trees and remove sources of infection.
• Malaria: mosquito nets, repellents, insecticides and removal of breeding habitats.
• Chlamydia and HIV: condoms, abstinence and screening. Avoid shared needles to reduce bloodborne HIV transmission.

Disease interactions

HIV → fewer functioning white blood cells → weaker immune defence → greater susceptibility to other infections → possible AIDS if untreated.

Certain HPV infections increase cervical cancer risk. The infection can spread; the cancer itself cannot.