Invasion – entry of pathogens into the body

Routes by which infectious organisms gain entry into the body include the skin, respiratory tract, gastro-intestinal (GI) tract and GU tract. There are fundamentally two ways in which infectious agents cross the physical and chemical barriers: either they are able to penetrate the intact barriers at one or more anatomical sites, or the physical barriers are damaged and breached, allowing entry of the organism.
Penetration of intact skin or mucosa
• Skin. Few organisms are able to penetrate intact skin. However, some parasites (e.g. hookworm) or their larvae (e.g. schistosoma) can do this. Other agents, such as wart viruses, set up infection in the skin and do not enter further into the body.

Entry of pathogens into the body. Insect bites, cuts, burns and animal bites breach the skin barrier, allowing entry of pathogens. Some parasites can penetrate intact skin while many pathogens penetrate intact mucosa of the respiratory, intestinal and genito-urinary tracts.

Mucosal sites of entry for pathogens

• Mucosa. Mucosa, being softer and damper than skin, are much more frequent sites of entry and all intact mucosa can be penetrated by some organisms. Examples are shown in table Pathogens can cross epithelia by passing through epithelial cells, as in the case of the meningococcus (a bacterium causing meningitis), or by passing between the epithelial cells, seen with Haemophilus influenzae.

Penetration of damaged skin or mucosa.
There are many ways in which skin or mucosa can be damaged, allowing entry of infectious organisms that could not cross intact skin or mucosa. Damage to skin is a particularly important route of infection and can occur in a number of ways:
• Burns. Burns, especially severe ones, pose a major risk for infection, particularly with Staphylococcus, Streptococcus, Pseudomonas and Clostridium tetanus.
• Cuts and wounds. These can allow entry of similar organisms to those seen after burns.
• Insect bites. Numerous infections are transmitted via insect bites. These include malaria, typhus and plague.
• Animal bites. Animal bites can provide direct transmission of infection, such as in rabies. Because they cause significant damage to the skin, bites can allow the entry of the same environmental pathogens as burns, cuts and wounds (see above).
• Human behaviour. Various aspects of uniquely human behaviour can result in the skin being penetrated. Sharing of syringes by intravenous (IV) drug users exposes them to risk of hepatitis and human immunodeficiency virus (HIV). A number of viral infections (hepatitis, HIV) have been transmitted by blood transfusion and blood products (e.g. factor VIII for haemophiliacs) before appropriate screening procedures were developed. Transplantation has also resulted in transmission of infection before the introduction of appropriate donor screening.

Damage to mucosa may not increase the likelihood of infection to the same extent as damage to the skin. However, physical or chemical damage may allow entry of some organisms (e.g. smoking increases the risk of respiratory bacterial infections). Furthermore, infection of the mucosa with a virus may cause damage and facilitate the entry of bacterial pathogens.