How are infectious diseases transmitted?

This page lists and explains ways in which infectious diseases are transmitted from person to person. The same principles apply to transmission of diseases between members of other species, e.g. of animals (and in some special cases between species).

Not all diseases are infectious diseases.
See causes of diseases for more about the different ways in which diseases can occur generally.

Infectious diseases are caused by pathogens.

Pathogens are disease-causing microbes.

Microbes can be either microorganisms, which are extremely tiny organisms - too small to be seen with the naked eye, or viruses - which are also extremely tiny but are not universally accepted as independently 'living' because they can only replicate inside living cells e.g. of a human or other animal.) There are many different types of microorganisms, some of which are useful while others cause disease.

There are different types of pathogens and different ways in which pathogens move around, including from person to person, and in some cases from people to other animals, and vice-versa.

The following table summarizes the main modes of transmission of infectious diseases:

Mode of Transmission Examples 1. Contact Some infectious diseases are spread via physical contact. These can be divided into transmission by: Direct physical contact with an infected person, e.g. skin contact, sexual contact, etc. Indirect physical contact with a contaminated surface or object. Examples: Examples of diseases spread by contact include via direct contact Herpes simplex (cold sores) Impetigo Leprosy and via direct or indirect contact Athlete's Foot (fungus) 2. Droplets A cloud of tiny water droplets is produced and released when people (and many other animals) cough or sneeze. If someone who has an infectious disease than can be transmitted via droplets sneezes and another person nearby inhales (breathes in) droplets infected with the pathogen causing disease in the person who sneezed, the person who inhaled the drops may* become infected by the same pathogen. Examples: Diseases spread via droplets: Common cold Flu (influenza) Measles Tuberculosis Chickenpox Whooping cough 3. Food Food should be prepared, stored (if it is stored between preparation and serving), served and eaten under hygienic conditions e.g. using clean surfaces, equipment and utensils. If hygiene isn't maintained then harmful ('pathogenic') bacteria might enter the food and breed within it. If so, the bacteria might continue to reproduce and cause disease within the bodies of people who eat the food. That is, they might* become ill due to food poisoning. Examples: There are many examples of diseases spread via contaminated food, e.g. Salmonella Campylobacter E-coli (Escherichia coli) Botulism (Clostridium botulinum) and others, some also spread via water. Note: It is, of course, also possible to cause food poisoning by deliberately putting harmful substances into food intended for human or animal consumption. In that case, the cause of the resulting disease would not be 'bad hygiene' but could be criminal activity. Note: It is, of course, also possible to cause food poisoning by deliberately putting harmful substances into food intended for human or animal consumption. In that case, the cause of the resulting disease would not be 'bad hygiene' but could be criminal activity. 4. Water In many places water intended for drinking (drinking water) is treated and tested to ensure that it is safe to drink, which includes it being free from pathogens. However, that is not always the case and some water considered safe for human consumption might become contaminated, e.g. the water supply in part of London that Dr John Snow found to be the cause of a cholera outbreak in 1854. Water that has been contaminated with sewage is likely to contain pathogens (disease-causing microbes) that could* cause disease in people who drink, or sometimes even just bathe in, the water. Examples: Diseases spread via contaminated water: Cholera Hepatitis A Dysentery Polio Typhoid various parasites and many others, including some also spread via food. 5. Wounds The functions of skin include protection against disease because the skin acts as a barrier to pathogens entering the body. However if the skin is damaged (resulting in an open wound), pathogens such as harmful bacteria could enter the body's tissues and possibly even the blood stream - via which pathogens might* be able to travel to infect other parts of the body. Examples: Diseases spread via wounds (e.g. broken skin): Warts Tetanus Septicemia MRSA (Methicillin-resistant Staphylococcus aureus) 6. Vectors In the context of causes of disease+, a vector is an organism, e.g. an animal, that does not cause disease itself but that transmits infection by conveying pathogens from one host to another. For example, an insect, that spreads disease by moving the relevant microbe to a person - who may then become infected, is a vector of that disease. In many cases the vector (animal) transmits disease by biting. Alternatively the vector could contaminate food or water, e.g. houseflies and cockroaches contaminate unprotected food by bringing microbes that could cause food poisoning. Examples: Diseases spread by vectors: Malaria (via mosquitos) Lyme disease (via ticks) Rabies (via any rabid animal e.g. dogs, foxes, raccoons, skunks, cats, bats - but most commonly dogs) Sleeping sickness (via tsetse fly) Plague (via fleas, to humans and rats) Japanese encephalitis (via mosquitos) + as opposed to, for example, in mathematics and physics - where a 'vector quantity' is something that has a direction as well as a magnitude, e.g. speed is just a number that indicates how fast something is moving while velocity is speed in a specific direction. + as opposed to, for example, in mathematics and physics - where a 'vector quantity' is something that has a direction as well as a magnitude, e.g. speed is just a number that indicates how fast something is moving while velocity is speed in a specific direction.

Note: The numbers (1.) to (6.) shown to the left of each of the methods of transmission of infectious diseases listed in the table above are for ease of reference to this list only. They do not indicate sequence, prevalence, importance or any other significance.

Many infectious diseases can be transmitted by more than one of the six modes of transmission listed above. For example, many bacterial diseases can be spread via contaminated food or contaminated water - many foodstuffs have high water content so there is sometimes very little distinction between 'food' and 'water' e.g. soup made using water or water used to wash food such as fruit.

Combinations of modes of transmission of infectious diseases

Transmission via Wounds and Vectors

There is an interesting over-lap between the categories 5. Wounds and 6. Vectors.
In the case of transmission of an infectious disease via an animal bite, the mode of transmission would is the vector because the pathogen entered the body via a break in the skin caused by the vector e.g. a mosquito bite. Nevertheless, especially in the cases of bites from larger animals such as mammals, the bite is also a wound. Theoretically, some diseases such as rabies can be transmitted, not only via animal bites, but also via scratches, abrasions or open wounds contaminated with saliva or nervous tissue from a rabid animal. So, although a bite or a scratch may be caused by the animal, if the person handled the animal or its tissues or bodily fluids with bare hands that had small unprotected open wounds from any other cause, it is thought possible that he or she could still contract the disease. In that case, the animal (vector) was still an essential aspect of the means by which the diseases reached the person because the rabies virus could not survive outside the animal for long, e.g. it could not survive in dried-up saliva. However, the person's wound also played an essential role in the transmission of the disease to the person.

Transmission via Food and Vectors

Many small animals e.g. cockroaches and other insects are attracted to human food, which they can also feed on. Especially if the same insect(s) that land on food have recently landed on human or animal excrement or other contaminated material, they can bring pathogens including various harmful microbes to the food. The food might then be contaminated with e.g. harmful bacteria. However, because microbes are so small the food wouldn't look any different after the insects had left. A person who ate food that had been contaminated in this way could become ill and would have received the disease (e.g. a form of food poisoning) via both the vector (e.g. insect) and the contaminated food.

* Even when there is a possibility of infection, the disease might not be transmitted effectively and cause the disease in the person who received it. This is because the person who received the pathogen might have immunity to it. Living a healthy lifestyle, including eating a balanced diet and taking appropriate exercise, increases the ability of the person's immune system to resist infection by the many pathogens encountered in everyday life.

Note: The above list of disease transmission methods is taught to secondary school age pupils in the UK. It is not the only way to break-down the different ways in which infectious diseases can be transmitted. For example, some medical sources classify transmission of diseases as occurring via droplet contact, direct physical contact, indirect physical contact, airborne transmission and faecal-oral transmission . This page may include more or less detail than required for any particular course. Check course requirements with a teacher, lecturer or course outline or syllabus.