Common pathogens and communicable diseases

In a nutshell

Communicable diseases are diseases caused by pathogens that can be spread between organisms. There are four types of pathogens: bacteria, viruses, fungi and protists. They all infect both plants and animals. Pathogens are spread via water, air, contact, animal vectors, food and body fluids.

Communicable diseases

Definition

Communicable diseases, sometimes called infectious diseases, are diseases that can spread between organisms. They are caused by pathogens.

Pathogens

Pathogens are microorganisms that cause disease and include bacteria, viruses, protists and fungi. 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 by 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 infection (STI) 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.

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, which 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 are able to live a normal life. If the infection is not controlled, the virus 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, which 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.

Transmission

Communicable diseases can be transmitted directly or indirectly.

Direct transmission

Direct transmission occurs when a disease is transmitted directly from one organism to another. This can be via droplets (coughing/sneezing), sexual intercourse or touching.

Example

HIV is spread via sexual intercourse.

Indirect transmission

This occurs when diseases are transmitted between organisms via an intermediate such as air, water or an animal vector.

Example

Malaria is transmitted via a mosquito vector.

Factors affecting transmission

There are other factors that affect the transmission of communicable diseases:

Factor	Example
Climate	Malaria is common in tropical countries as they are humid and hot: this is ideal for breeding mosquitos.
Overcrowding	Respiratory diseases such as tuberculosis are spread via air droplets and can spread easily in overcrowded living conditions, as people are in close proximity.
Social factors	The risk of infection by HIV is higher in places with limited access to good healthcare and good education.