Microorganisms that cause disease in humans are called pathogens. Bacteria, fungi, protozoa and viruses fall into this category. These pathogens are able to enter the body and cause disease. This invasion and the responding symptoms are called an infection. Infections may be localized and restricted to a tiny area, or systemic, with a broader area being affected.
Bacteria are found all over. They are single-cell organisms that consider the human body to be ideal living conditions. There are normal flora, or bacteria that are useful to the body, as well as pathogens, which cause disease. The largest group of pathogens, bacteria are classified based on shape. Round bacteria, or coccus, are capable of causing meningitis and pneumonia. Bacillus, or rod-shaped bacteria, cause tuberculosis and tetanus, while curved rod-shaped bacteria cause cholera and syphilis. All bacteria have a cell wall that protects the microorganism and the ability to form spores that allow them to stay alive in very harsh conditions.
Fungus is an organism that is plantlike and grows best in dark, moist places. Mold and yeast are two forms of fungi. Infections that are caused by fungi are called mycotic. These are usually localized, like athlete's foot, thrush and ringworm. Systemic fungal infections, which are difficult to cure and frequently life-threatening, are possible but they are rare.
Protozoa, such as amoebas, ciliates and sporozoa, are animal-like single cell organisms. They are generally found in bodies of water and soil. They are capable of causing dysentery, which is a condition characterized by severe diarrhea, usually seen after a person drinks contaminated water. Malaria is caused by a sporozoa that is carried by the mosquito.
The smallest of the pathogens, viruses are not cells. They are simply RNA or DNA surrounded by a shell of protein. They are parasites, which can only live within living cells. Influenza, mumps and measles are some of the diseases caused by viruses. Some viruses, such as herpes simplex, can remain dormant for years and flare up periodically for years. Others, such as the common cold, are able to terminate without any medical treatment. Some viruses, such as HIV (the virus which causes AIDS), rabies and hepatitis, are very serious and can be fatal.
Spread of Infection
Germs move in and out of the body, spreading infection. They enter the body through a portal of entry, such as the eye, skin, digestive tract or respiratory system. They may also enter through an injury such as a cut or bite. Microorganisms leave the body through a portal of exit, usually through the respiratory or digestive tract, or through sexual contact. Pathogens sometimes spread through person-to-person contact, such as catching a cold from a co-worker. It is also possible for them to spread through environment-to-person contact, such as developing dysentery after drinking contaminated water, or insect-to-person contact, like contracting malaria from a mosquito that carries the disease.
Hand washing is the most important thing you can do to prevent the spread of disease. Health care workers should always wear gloves when handling blood or body fluids. It is also important to clean soiled clothing and linens to prevent the spread of disease. Taking care to only eat and drink foods that are not contaminated is another way to prevent illness.