Communicable diseases

In a nutshell

Communicable diseases are diseases caused by pathogens that can be spread between organisms. There are four types of pathogen: bacteria, viruses, fungi and protists. They all infect both plants and animals. Pathogens are spread via water, air, contact, animal vectors, food, body fluids and soil. It is possible to reduce the spread and prevent disease by implementing measures such as good hygiene and chemical control.

Communicable diseases

Communicable diseases, sometimes called infectious diseases, are diseases that can spread between organisms. They are caused by pathogens. Pathogens are microorganisms that cause disease and include bacteria, viruses, protists and fungi.

Pathogens

Once inside an organism, pathogens can rapidly multiply and the infected organism will normally start to show symptoms of the disease. There are four types of pathogen:

Pathogen	Description
Bacteria	These are very small cells that reproduce very rapidly. They cause illness by producing toxins that damage your cells and tissues.
Viruses	These are not cells. They are very tiny (smaller than bacteria) and replicate themselves inside the infected organism's cells. This causes the organism's cells to burst and release the viruses.
Fungi	Fungi vary in size. Some are single-celled and others have bodies that are made up of thread-like structures called hyphae. They produce spores which help fungi to spread to other plants and animals.
Protists	These are eukaryotic, single-celled pathogens that vary in size. Protists that cause diseases are usually called parasites.

Bacteria

Salmonella

Salmonella causes food poisoning. Symptoms include fever, stomach cramps, vomiting and diarrhoea. It produces toxins that cause these symptoms.

It is spread by either eating food that already contains the bacterium, such as a chicken that caught Salmonella when it was alive or eating food made in an unclean kitchen. In the UK, most chickens are vaccinated against Salmonella, this is a method of disease control.

Gonorrhoea

Gonorrhoea is a sexually transmitted bacterial infection that is spread through unprotected sex. Symptoms include pain when urinating and a thick yellow-green discharge from the vagina or penis. Gonorrhoea used to be treated with an antibiotic called penicillin, however, new gonorrhoea strains are resistant to penicillin and some types of antibiotics. This is called antibiotic resistance.

To prevent the spread of gonorrhoea, people should use barrier methods of contraception such as a condom and they should be treated with other types of effective antibiotics.

Note: Bacterium is the singular version meaning one. Bacteria is plural.

Viruses

Measles

Measles is a very infectious viral disease spread by droplets from an infected person's sneeze or cough. Symptoms include a red skin rash and fever. It is often a life-threatening infection if people develop complications, this is why people are vaccinated against measles in early childhood.

Human immunodeficiency virus (HIV)

HIV is spread by exchanging bodily fluids, usually through sexual contact. HIV initially causes flu-like symptoms for a few weeks and then no symptoms for several years. The virus enters lymph nodes and attacks a person's immune cells. This causes their immune system to be weak and increases their susceptibility to other diseases.

HIV can be controlled with antiretroviral drugs that stop the virus from replicating in the body, people who take these drugs can live a normal life. If the infection is not controlled, the HIV will attack the majority of a person's immune cells and cause acquired immune deficiency syndrome (AIDS) which is the late stage of HIV infection.

Tobacco mosaic virus (TMV)

TMV is a virus that affects many different species of plants including tomatoes. It causes their leaves to become mottled and discoloured, giving them a mosaic pattern. The discoloured leaves have less chlorophyll to absorb light during photosynthesis, this means the plants cannot produce enough food to grow.

Fungi

Rose black spot

Rose black spot is a plant infection caused by a fungus. It causes purple or black spots on the leaves of rose plants. The leaves then turn yellow and drop off. This means less photosynthesis happens so the plant doesn't grow very well. It is spread via water or wind and can be treated with fungicides.

Protists

Malaria

Malaria is caused by a protist known as Plasmodium. Part of the protist's life cycle takes place inside a mosquito and the mosquitos are the disease vectors. Malaria causes repeating episodes of fever and can be fatal.

Disease transmission

Pathogens can be spread in different ways.

Mode of transmission	Description	Example(s)
Water	Some pathogens are spread by drinking or bathing in contaminated, dirty water.	Cholera is a bacterial infection that causes diarrhoea and dehydration. It is spread through drinking water containing diarrhoea of infected people.
Air	Some pathogens are carried in the air	Erysiphe graminis is a fungus that causes white, fluffy patches to appear on the leaves of barley plants. This affects the plant's ability to do photosynthesis which decreases the yield of barley plants for barley farmers. It is spread by spores that are blown between plants in the wind.
Air	Airborne human pathogens are carried in droplets that are produced when you cough or sneeze, this causes other people to breathe them in.	Influenza virus (flu) and SARS-CoV-2 (COVID-19)
Contact	Some pathogens are spread by touching contaminated surfaces.	Tobacco mosaic virus in plants is spread by infected leaves touching uninfected leaves.
Contact		Athlete's foot is a fungal infection that makes the skin itch and fall off, it is spread by touching the same shower floors and towels as an infected person.
Body fluids	Some pathogens are spread via bodily fluids such as blood, breastmilk and semen.	HIV is spread through the exchange of body fluids.
Animal vectors	Animals that spread diseases are called vectors.	Malaria is spread via the mosquito vector. The mosquito picks up the protist when it feeds on an infected animal, it is then spread when the mosquito feeds on another animal. By feeding, the protist is inserted directly into the animal's blood vessels.
Soil	Some pathogens live in the soil, so plants that are in the contaminated soil get infected.	The bacterium Agrobacterium tumefaciens causes crown gall disease. It can live freely in soil and on the roots of some plants, if it enters the plant it can cause growths or tumours called galls on stems and branches. The galls can damage the plant tissue, restricting the flow of water through the plant. The plant becomes weaker and can die.
Food	Some pathogens are spread via contaminated food. If foods are kept too long or not cooked properly, bacteria can cause food poisoning.	Salmonella bacteria are found in some foods. Example Raw meat.

Reducing and preventing the spread of disease

There are some things that can reduce and prevent the spread of communicable diseases.

Social and economic factors

Certain social and economic factors increase or decrease the spread of disease.

Factor	Explanation
Space	The transmission of communicable diseases increases when lots of people are crowded together in small living spaces because it is easier for the pathogen to pass between people. Example Coughing
Poor diet	Eating a poor diet can increase the risk of infection as the immune system will be weakened by not getting the right nutrients.
Healthcare	Having access to healthcare means people are more likely to get diagnosed and receive effective treatment. This reduces the chance of them infecting others.
Health education	Good education gives people better knowledge of how diseases are transmitted so they can avoid catching diseases in the first place.

Reducing and preventing disease in humans

Early detection means early treatment which can limit the spread of the disease.

Factor	Explanation
Being hygienic	Using simple hygiene measures such as washing your hands after sneezing can stop the spread of disease.
Destroying vectors	Getting rid of organisms that spread disease can prevent the disease from being passed on. Vectors like mosquitos can be killed using insecticides or by destroying their habitats so they can no longer breed.
Isolating infected individuals	This stops the communicable disease from being passed on to another person.
Vaccination	Vaccinating people against communicable diseases normally means they cannot develop the disease and pass it on.
Early detection	Early detection means early treatment which can help limit the spread of infection.

Reducing and preventing disease in plants

Plants are an important food source and plant diseases reduce crop yield and biodiversity of ecosystems, this is why plant diseases need to be avoided.

Method	Description
Regulating movement of plant material	This makes sure infected plants do not come into contact with uninfected plants.
Destroying infected plants	This stops the spread of disease.
Crop rotation	Most pathogens are specific to the type of plant, so changing the types of plants that are grown stops the pathogens from becoming established in an area.
Chemical control	Chemical controls such as fungicides can be used to kill fungal pathogens. Fungicides can be used preventatively by coating bulbs or seeds before they are planted.
Biological control	Cown gall disease can be prevented by dipping the roots of plants into a suspension containing a similar bacteria to Agrobacterium tumefaciens. This similar bacteria isn't infectious and prevents infection by producing an antibiotic.

Note: Binomial names of pathogens and other organisms are always italicised or underlined e.g., Agrobacterium tumefaciens or Homo sapiens (unless the rest of the sentence is italicised, in which case the binomial name would be in normal text).