EXAMPLES OF THE LIFESTYLE OF PATHOGENIC ORGANISMS.

Measles.
The measles virus enters the body through the respiratory tract. It then travels to local lymph nodes and lymphoid tissue located in the mucosa. After a few days the virus spreads to other lymphoid tissue, including the spleen, where it begins to replicate. After a week or so, large quantities of the virus spread via the bloodstream to epithelial sites throughout the body. The presence of large amounts of virus at these sites gives rise to the various symptoms seen in measles. Virus in the respiratory tract causes runny nose and coughing. There is inflammation of the conjunctiva, and the presence of the virus in the skin causes the characteristic rash seen in measles.
Typhoid.
If the Salmonella typhi bacterium is ingested and the dose is big enough, some bacteria will survive the acid environment of the stomach and enter the intestine. Bacteria penetrate the gut mucosa through specialised lymphoid structures known as Peyer’s patches and spread to the intestinal lymph nodes, where they proliferate in macrophages. Eventually the organisms reach the bloodstream, where they spread mainly to the liver, bone marrow and spleen, where they continue to multiply. This results in a further large increase in bacterial numbers and subsequent spread of the organism to other tissues such as the kidney and the gall bladder via blood or the biliary tract. The bacteria can also spread to the brain, heart and skin. The bacteria then invade the intestinal tract in much larger numbers than seen with the original infection and cause inflammatory lesions in the Peyer’s patches, which may result in ulceration of the intestinal wall. The presence of the bacteria in other sites may cause meningitis, osteomyelitis, endocarditis and rashes.