Spread of pathogens

The way in which organisms spread through the body is influenced to some extent by whether they live intracellularly, extracellularly or both. Organisms that live extracellularly are able to spread via body fluids such as blood. However, even organisms that replicate intracellularly may be able to leave the cell and spread via an extracellular route. Organisms can spread in the following ways:
• Cell to cell contact. Many organisms, especially viruses, spread directly from cell to cell with essentially no extracellular component to their lifestyle. These pathogens tend to cause localised infections such as seen in influenza, where only the respiratory tract is infected. However, localised infections can still cause widespread symptoms, so that ’flu causes headache, fever and muscle-ache.
• Via blood and lymphatic vessels. The commonest, and fastest, way in which pathogens can spread through the body is via the bloodstream. Since all organs and tissues require a blood supply, microorganisms in the blood have the potential to spread to all sites. However, individual pathogens show a preference to localise in particular organs or tissues that may differ from pathogen to pathogen.
The lymphatic vessels form a circulatory system that parallels that of the blood. There are important differences between the two systems, however. The circulation of the lymphatic fluid is maintained not by the heart but by the movement of the muscles surrounding the lymphatic vessels; thus lymphatic fluid flows at a much more sluggish rate than blood. Moreover, tissue fluid can drain directly into lymphatic vessels. Organisms can easily enter lymphatic vessels draining the site of infection, where they will be conveyed to the local lymph nodes.
• Spread via body cavities. Microorganisms that have infected one organ in a body cavity such as the peritoneum may occasionally spread via the cavity to other organs located within it.
• Spread via nervous system. This is a particularly important route of spread for certain viruses. Viruses can spread via peripheral nerves to the central nervous system (CNS) or vice versa. In some instances this route of spread allows the virus to become more widespread within the nervous system where it resides and causes disease (e.g. herpes simplex virus). In other cases the virus travels via nerves to infect other organs. The rabies virus infects the salivary glands in this way, enabling the virus present in the saliva to be transmitted via a bite.