However, whether it is obvious or not, each disease-producing organism has its own portal or portals of entry as well as a means of escape from the host.
Microorganisms enter the body through the following areas:
1. Respiratory tract:
By way of the nose and mouth. This is the portal of entrance for microbes causing respiratory diseases such as the common cold, measles, pneumonia, and tuberculosis.
2. Gastrointestinal tract:
By way of the mouth. Examples include agents responsible for typhoid fever, paratyphoid fever, dysentery, cholera, polio, and hepatitis, as well as many food borne illnesses such as botulism and staphylococcal food poisoning.
3. Skin and mucous membranes:
Although the skin provides an effective barrier, some organisms appear capable of penetrating the intact skin; in addition minor breaks frequently occur, allowing the entrance of many organisms.
The staphylococcus that causes boils and furuncle frequently enters by this route; however, streptococci may also cause spreading skin intections. Tularemia, leptospirosis, and anthrax are examples of severe systemic diseases usually contracted through the skin from handling infected animals or animal products.
4. Genitourinary system:
The mucous membranes of the genital tract are the site for invasion by agents causing venereal diseases such as gonorrhea and syphilis. In addition, the urinary tract may be infected by microorganisms originating in the blood and infecting the kidney or by the introduction of organisms into the bladder during catheterization.
Those organisms that must be introduced directly into the blood to cause disease usually are transmitted from one individual to another by insects that penetrate the protective skin barrier with their bites. In addition to those mentioned earlier, our advancing civilization has added another way for direct blood inoculation; inadequately sterilized syringes and needles.
Many cases of hepatitis have been transmitted by using whole blood from individuals who are hymptomatic carriers of the hepatitis virus in their blood and acquired immune deficiency syndrome (AIDS)) has also been transmitted by whole blood or blood products.
The portals of exit for a disease agent are usually the same as their portals of entry. Thus, diseases of the respiratory tract are spread by way of secretions and excretions of the respiratory tract and mouth.
Similarly, microorganisms causing enteric infection leave the body by the intestinal tract and are spread through fecal contamination. Skin or wound infections may be spread by drainage from these areas either directly to another person or through contamination of some inanimate object.
Blood infections, which are spread by insects or contaminated needles or syringes, usually leave the individual in a similar manner, through direct contact with a needle or syringe during the withdrawal of blood or by the ingestion of microorganisms by a biting insect.