Research for infection protection

Transmission paths

Transmission of pathogens to humans occurs through many different routes. However, many of the clinically relevant pathogens have mainly four modes of transmission.


Human pathogen transmission

Infection via droplets or particles in the air

Droplet and airborne infections are part of the direct transmission path, occurring via infected humans or animals. Pathogenic droplets that are produced during speaking, sneezing or coughing reach the mucous membranes of the upper respiratory tract, where they propagate. The droplets are larger than 100 µm, which roughly equals the diameter of a human hair. The droplets can only travel short distances. Flu infections and influenza virus are transmitted this way, especially during the cold season. When a droplet nucleus, which measures five µm only, is dispersed over long distances through the air, this is referred to as airborne transmission. For example, Mycobacterium tuberculosis, the cause of tuberculosis, is passed on like this.

Contact infection and smear infection

Pathogens are transmitted directly or indirectly, primarily via hands. Direct contact transmission occurs when there is direct physical contact between an infected person or animal and a non-infected person. There is no intermediate host. Herpes simplex virus (HSV), which causes herpes on lips, uses this path to infect the host.

Indirect contact infection occurs between a non-infected person and the environment, for example objects. The pathogens reach the host, for instance, via open wounds or mucous membranes. One form of indirect contact infection is the smear infection, where pathogens are transmitted via the faecal-oral route (i.e. via the excretions of infected persons). One example of this transmission path is the norovirus, which may lead to severe diarrhoea.

Infection via blood and tissue

The transmission of pathogens from the environment to humans is referred to as indirect contact infection, e.g. via needle prick injuries. Hepatitis B, for instance, is passed on this way. However, disease-causing microorganisms may also be transmitted via body secretions, e.g. saliva, sweat, pus, or semen. HIV, for example, is transmitted through sexual contact.

Indirect contact infection may also occur through bites or stings of blood-sucking insects. From an infected host animal, the pathogen is passed on to humans via a transmitter and thus reaches their bloodstream. The transmitter, also referred to as vector, does not become infected. One example is the Black Death: the bacterium Yersinia pestis colonising rats is transmitted to humans via rat flea.

Infection via contaminated water and food

Indirect contact transmission occurs when there is no direct contact between carriers of infection and newly infected persons, for example, via contaminated water or contaminated food. This mode of transmission often occurs in areas with poor hygiene: bacteria, for instance, reach the drinking water via excretions or colonise animal products. The cause of typhus, Salmonella typhi, is transmitted this way. Another example is the bacterium Vibrio cholerae, the elicitor of cholera.