Koch's postulates are guidelines for determining that a specific microorganism causes a specific disease. Koch's postulates are as follows: 1. The same pathogen must be present in every case of the disease. 2. The pathogen must be isolated from the diseased host and grown in pure culture. 3. The pathogen from the pure culture media must cause the disease when it is inoculated into a healthy, susceptible laboratory animal.

4. The pathogen must be isolated from the inoculated animal and must be compared to the original pathogen.

Microorganisms can enter the body in a variety of ways. They can penetrate the mucous membranes of the respiratory, gastrointestinal, genitourinary systems and the conjunctiva, the membrane that covers the eyes. Microbes can easily be inhaled through the respiratory tract. They can also be ingested in contaminated food. Some microorganisms enter the body through the skin. Some microbes, such as fluke worms, can burrow through the skin into the bloodstream. Bacteria, viruses, and protozoan can go through the skin via a bite, cut, or other wound; this is called parenteral entry.

The cardiovascular system is responsible for circulating blood throughout the body to deliver cells nutrients and to transports wastes away. It is made up of the heart, blood, and blood vessels. The heart is a muscle that pumps the blood throughout the body. Blood is made plasma, red blood cells, or erythrocytes, and white blood cells, or leukocytes. Plasma transports dissolved nutrients to cells and takes away wastes. Red blood cells carry oxygen to cells. White blood cells play a role in the body's defense against foreign microorganisms. Arteries, veins, and capillaries comprise the network of blood vessels in the body, which transport blood away from the heart, bring blood back to the heart, and distribute materials to and from blood to cells respectively. The cardiovascular system has no normal microbiota.

One of the more well known viral diseases is infectious mononucleosis, or commonly called mono. This disease is transmitted through the ingestion of infected saliva, whether by kissing or sharing drinks. The virus which causes mono, the Epstein-Barr or EB virus, reproduces in the parotid glands, which produce saliva. The EB virus has an incubation period of 4 to 7 weeks before symptoms begin to appear. Symptoms include fever, sore throat, buboes, or swollen lymph nodes, in the neck, and a general feeling of weakness. Symptoms, however, do not appear when young children contract the disease. The EB virus infects solely B cells; the infection causes them to transform into plasma cells. Cytotoxic T cells consider these plasma cells to be foreign invaders and attack them. This causes the symptoms in patients. Infectious mononucleosis is largely non-fatal; death can be caused if the enlarged spleen associated with the disease is ruptured during vigorous activity. There is no specific treatment for mono. Recovery is complete within a few weeks with the patient having complete immunity to the disease thereafter. Testing for mono is comprised of the detection of antibodies against the EB virus. Mono can be found all over the world including the United States.

Burkitt's lymphoma is another disease that is related to the Epstein-Barr virus. It is a cancerous tumor of the jaw that is most commonly found in children. Although the cancer is not directly caused by the EB virus, research shows that it is closely associated with the tumor. Scientists still do not know how the virus could start the tumor. Burkitt's lymphoma occurs as mosquitoes infect a person with malaria. The malarial infection in turn weakens the immune system, making the person more susceptible to the EB virus, which is commonly found. Burkitt's lymphoma can only be found in Africa, where malaria is endemic. However, AIDS patients have also been known to contract the disease because of their weakened immune systems

One emerging viral hemorrhagic fever is caused by the now well publicized ebola virus. Caused by the ebola filovirus, ebola first appeared in an outbreak in Africa in 1976. The natural host is unknown, but the disease is spread by contact with blood, often through unsterilized needles. Ebola has a staggering mortality rate of ninety percent.

Some common words found in the essay are:
Plasmids DNA, Treatment Chaga's, Systems Koch's, United Burkitt's, Epstein-Barr EB, South America, Southeast Asia, Ova Schistosoma, America Africa, blood cells, yellow fever, immune system, host cells, eb virus, red blood, white blood cells, lymph nodes, red blood cells, white blood, disease caused, hemorrhagic fever, America Caribbean, cells white blood, blood cells leukocytes, respiratory gastrointestinal genitourinary,
Approximate Word count = 2844
Approximate Pages = 11 (250 words per page double spaced)