1.2.4  Modes of transmission

Once an infectious agent leaves a reservoir, it must get transmitted to a new host if it is to multiply and cause disease. The route by which an infectious agent is transmitted from a reservoir to another host is called the mode of transmission. It is important for you to identify different modes of transmission, because prevention and control measures differ depending on the type. Various direct and indirect modes of transmission are summarised in Table 1.3 and discussed below it.

Table 1.4  Summary of different modes of transmission.

Mode of transmissionSub-types of transmission


Sexual intercourse


Direct projection of droplets

Across the placenta





Direct modes of transmission

Direct transmission refers to the transfer of an infectious agent from an infected host to a new host, without the need for intermediates such as air, food, water or other animals. Direct modes of transmission can occur in two main ways:

  • Person to person: The infectious agent is spread by direct contact between people through touching, biting, kissing, sexual intercourse or direct projection of respiratory droplets into another person’s nose or mouth during coughing, sneezing or talking. A familiar example is the transmission of HIV from an infected person to others through sexual intercourse.
  • Transplacental transmission: This refers to the transmission of an infectious agent from a pregnant woman to her fetus through the placenta. An example is mother-to-child transmission (MTCT) of HIV.

Indirect modes of transmission

Indirect transmission is when infectious agents are transmitted to new hosts through intermediates such as air, food, water, objects or substances in the environment, or other animals. Indirect transmission has three subtypes:

  • Airborne transmission: The infectious agent may be transmitted in dried secretions from the respiratory tract, which can remain suspended in the air for some time. For example, the infectious agent causing tuberculosis can enter a new host through airborne transmission.
  • Vehicle-borne transmission: A vehicle is any non-living substance or object that can be contaminated by an infectious agent, which then transmits it to a new host. Contamination refers to the presence of an infectious agent in or on the vehicle.
  • Vector-borne transmission: A vector is an organism, usually an arthropod, which transmits an infectious agent to a new host. Arthropods which act as vectors include houseflies, mosquitoes, lice and ticks.

Arthropods are invertebrates (animals without backbones), such as insects, which have segmented bodies and three pairs of jointed legs.

  • Can you suggest some examples of vehicles that could transmit specific infectious agents indirectly to new hosts?

  • You may have thought of some of the following:

    • Contaminated food, water, milk, or eating and drinking utensils. For example, the infectious agent of cholera can be transmitted to a person who eats food or drinks water contaminated with faeces containing the organism.
    • Contaminated objects such as towels, clothes, syringes, needles and other sharp instruments. For example, sharp instruments contaminated with HIV-infected blood can transmit HIV if they penetrate the skin of another person.
    • Soil is a vehicle for some bacteria. For example, a person can be infected with bacteria that cause tetanus if contaminated soil gets in through broken skin.
  • Can you think of a vector-borne disease mentioned several times in this study session?

  • Malaria is transmitted by mosquito vectors.

1.2.3  Route of exit

1.2.5  Route of entry