Too small for us to see without the help of a microscope.

Microorganisms

Microorganisms are living organisms, most of them unicellular creatures that can be seen only with a microscope. Humans need them to live. They help us digest food and enable the normal development of our immune system. Microbes include viruses, bacteria, fungi and parasites, which can cause disease when our immune system can't fight them off.

Microorganisms are everywhere in our environment, on and in our bodies. In most cases they do us no harm. In fact, under normal conditions, our bodies house entire populations of microorganisms, kept in balance and harmless (these are termed endogenous populations). These microorganisms are vital for our bodies; they form an essential system that helps protect our body.

Actually, a symbiotic relationship between the host’s body and the microorganisms is formed. In this symbiosis, the microorganisms get to live in a comfortable environment and the host is not only unharmed, it benefits in the form of protection from other microorganisms.

General protection:
 The protection afforded to our bodies by such an endogenous population is the result of competition over resources: competition between native microorganisms that normally inhabit our bodies, and alien microorganisms that threaten to invade our bodies. Among these aliens there may be pathogenic (disease-causing) microorganisms.

Benefit:
 Men and other animals benefit directly from certain microorganisms located in specific areas of the body which help in various metabolic activities.

 A microorganism that causes disease:

During the course of an animal or human's life, there are changes in the body's content of endogenous microorganisms. These changes are the effects of changes in nutrition, environment, way of life and other similar factors. In cases in which the balance between the body and the endogenous microbes is harmed, a disease may erupt, as a result of a pathogenic microbe.

All of the microorganisms that live with us and in our environment are on the lowest branch of the evolutionary tree. Nevertheless, what they lack in sophistication and superior development, they overcome by tremendous versatility, an ability to multiply rapidly, and, for some microbes, an incredible potential to mutate. Some pathogens reemerge or evolve to become even more pathogenic.

Despite of all of our knowledge of pathogenic microorganisms, outbreaks of the diseases they cause still occur.

Some microorganisms are helpful to humans, but some are harmful. The microorganisms below are disease-causing microbes.

It is usually important to know the specific microorganism that causes an illness. Many different microorganisms can cause a given condition, and the treatment is different for each of them. There are many ways to identify microorganisms. Despite the development of rapid identification systems, direct microscopic examination of samples taken from the site of infection is often the most rapid method of identifying microorganisms capable of causing disease.

However, the microorganisms must be of sufficient size and number to be seen with a regular microscope. Sometimes microorganisms can be seen with a microscope and recognized by characteristic shapes and colors. Usually, however, the microorganisms are too few or too small to see, so they may be grown in the laboratory until there are enough of them to be recognized using chemical tests.

The process of growing the organism is called a culture. Many microorganisms can be grown this way.

Cultures can also be used to test the sensitivity of microorganisms to various antibiotics, which can help a doctor determine which drug to use in treating an infected person. This strategy is particularly important because microorganisms are constantly developing resistance to antibiotics that were previously effective.

Microorganisms and other disease-causing organisms, pathogens, come in five different types: bacteria, fungi, protozoa, helminths, and viruses.

In this unit we shall focus on two types of microorganisms: Fungi and Bacteria

ü  A person is exposed to pathogenic fungi or bacteria

ü  Fungi or bacteria that live in the human body (endogenous) become pathogenic

ü  The humane immune system is not acting against the pathogen fast enough

What are microorganisms?
Types of Infectious Organisms
Biology of Infectious Disease