Extrinsic diseases are those diseases considered to be related to microbes, infectious, contagious, and communicable. Some contagious diseases are considered exogenous which means the microbes come from the outside. “Exo” means from outside. You can get a microbe horizontally from another adult. Vertical transmission is from parent to child or child to parent. Some microbes are associated with and transmitted with sex cells like the sperm and egg. Other vertical transmission of microbes might be in-utero across the placenta; still other children may contract a microbe during the delivery and birth process; and young children do catch and pass disease microbes from and to their parents. You can get extrinsic diseases from your peers or other adults or children.

Still other contagious diseases may be endogenous—you actually give yourself the disease. When your body defenses are weakened or compromised, your flora may actually produce disease in you. Your normal flora is part of you, just like the color of your eyes; and if you get worn down, then the Streptococcus in your throat may become pathogenic and you develop sore throat. Another organism that may remain “latent or at rest” in nerve masses near the base of your spine is the chickenpox virus. Once again if you are under a lot of stress, a disease may occur. What do you call the condition or disease producing burning sensations and pain along abdominal nerves? Shingles is the disease resulting from the formerly dormant chickenpox virus. Your body probably never rid itself of all the viruses from chickenpox, and when a major test in microbiology causes you extreme stress, the virus may show disease symptoms. Another virus that may survive in your body is the herpes virus. If you have ever had genital herpes or herpes simplex (which causes cold sores), it is possible that during stress the symptoms of each may occur or flare-up again.

Extrinsic diseases are grouped together in a wide variety of ways. The diseases may be grouped according to causative microbe so some textbook authors have chapters such as bacterial diseases, viral diseases, etc. in their books. If an author of a textbook is a medical doctor, she or he may group extrinsic diseases in groups related to system affected—breathing system diseases, reproductive diseases, urinary diseases and others. Still other groups of extrinsic diseases may be foodborne, water borne, insect carried, etc.