Transmission and spread of pathogens

To enter cells or tissue, multiply and subsequently cause an infection, bacteria usually have to be transmitted to special sites within the host. Pathogens are mostly spread through direct contact with an infected person via droplet infection, smear infection (fecal-oral transmission) or sexual contact. Indirect transmission takes place via contaminated hands and non-living objects (especially in hospitals), and via contaminated food and drinking water as well as via airborne transmission. Some diseases are transmitted by special carriers, so-called vectors, such as blood sucking flies.

All injuries, even minor wounds of the intact natural protective skin and mucous membrane barriers, facilitate the invasion of possible pathogens.

Spread of infection within the host

Some possible pathogens belonging to normal microbial flora can cause endogenous infections in individuals with impaired immune responses and underlying medical conditions. Furthermore, some of these usually beneficial bacteria can cause infections if they leave their natural environment and gain access to other organs or tissues. Once pathogens have invaded the host’s tissue, they can cause a local infection or a general infection by spreading throughout the whole body. Continual influx of bacteria into the blood may result in sepsis.

Types of infection

A primary infection is the initial infection, which often is followed by a secondary infection. A viral common cold infection is often followed by an acute bacterial sinusitis. A secondary infection is usually caused by opportunistic pathogens that take advantage of the weakened host. A superinfection or overgrowth is a type of secondary infection. It may occur when the natural flora is disturbed, resulting in uninhibited growth of possible pathogens.

Infections may be caused by one single pathogen, but frequently by several as well (mixed infection). Infections can be community-acquired or hospital-acquired (nosocomial). Nosocomial infections are often more difficult to treat because pathogens are usually resistant to treatment and the patient´s immune condition is weakened.