Infectious disease results from a competition for supremacy between the parasite and the host. If the parasite overcomes the host, there is a change in the general state of good health and disease develops.
Several contributing factors are involved in the establishment of infectious disease. These factors determine whether the infecting organism will survive in the body.
Portals of entry. In order for a pathogen to gain access to the host, the pathogen must pass through a portal of entry. One of the most common portals of entry is the mucous membranes, especially those of the respiratory, gastrointestinal, and urogenital tracts. Another important portal of entry is the skin. Penetration of the skin occurs during a wound or by a hair follicle. When microorganisms penetrate below the skin, the portal of entry is said to be the parenteral route.
Dose. The dose of an organism refers to the number of microorganisms required to establish an infection. For some diseases, such as typhoid fever, the dose is a few hundred bacteria. For other diseases, such as cholera, the dose may be several million bacteria. The dose may be expressed as the LD 50 , which refers to the dose of microorganisms that will kill 50 percent of the hosts it enters.
Invasiveness. Invasiveness is a property that encourages disease because it refers to the ability of pathogens to penetrate into the tissues. Those organisms that cause intestinal ulcers, such as Entamoeba histolytica, penetrate the tissue effectively. Tissue invasion often begins with adherence, the ability of pathogens to attach to the tissue by using structures such as pili. The presence of a capsule or glycocalyx encourages adherence because they are composed of sticky materials.
Capsules. Microorganisms that possess capsules are able to resist host defenses by interfering with phagocytosis. Normally, the body uses white blood cells to engulf and destroy pathogens. However, toxic substances in the capsule are able to destroy the white blood cells before the white blood cells perform phagocytosis. The organism of pneumonia Streptococcus pneumoniae is well known for the toxic materials in its capsule. Many other pathogens also possess capsules.