What Is an Infectious Disease?
An infectious disease is caused by organisms that enter, live in, and multiply within the body. These organisms are called microorganisms because of their size; they cannot be seen by the naked eye. Not all microorganisms are bad for your health. In the human body there are many types of microorganisms that cause no problems, and often even help. These microorganisms are helping the body with normal functions and aid in the defense against pathogens. It is important to know what an infectious disease is, but knowing what causes the infection is as important. So, a question to address is: what are pathogens?
Pathogens
Organisms that cause disease are called pathogens. Pathogens are not normally found in the body. Pathogens can be categorized into four main groups; they are bacteria, viruses, fungi, and protozoans.
Bacteria are prokaryotic microorganisms that lack a membrane bound nucleus and organelles. They consist of one cell (unicellular). Bacteria are spherical, rod-shaped, or spiral-shaped. Bacteria are found all environments. They can be helpful or harmful. Bacteria are helpful in the environment by acting as decomposers and by being nitrogen-fixers. They also aid in the production of food.
Viruses are microscopic particles that invade the cells of plants, animals, fungi, and bacteria. Viruses always destroy the cells they invade, and only multiply with a living cell. A virus is not a cell. The composition of a virus is of a core genetic material surrounded by a protein coat. The protein coat protects the genetic material and enables the virus to invade its host cell.
Fungi are simple organisms that are related to molds. Many fungi are free-living in soil or water; others form parasitic or symbiotic relationships with plants or animals, respectively.
Protozoans are a collection of single-celled eukaryotic organisms with a well-defined nucleus). As such, they are among the simplest of all living organisms. Most are invisible to the naked eye. Many are symbionts of other organisms, and about one-third of the living species are parasites1. Symbiont is an organism that is associated with another in a mutually beneficial relationship.
The students have learned about what an infectious disease is and how pathogens are related to infectious diseases in the form of a lecture and discussion. Now, it is time to explore the how the body protects itself from infectious diseases that are caused by these pathogens. The body's system of protection is called the immune system.