How are infectious diseases transmitted?
Pathogens can be transmitted via:
- Contact (direct or indirect)
- Droplets e.g. released by sneezes
- Wounds 'direct contact' but skin must be broken
See the table below for notes about each of the above modes of transmission of diseases.
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:
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.