☆ bacteria, viruses, fungi, protozoa, and parasites are capable of causing disease in the human body. These disease-causing organisms are known as pathogens. An
infection is the invasion of the body by pathogens that then cause disease symptoms .
1) Bacteria: Bacteria are single-celled organisms that have a cell wall in addition to the cell membrane that our human cells possess. (This is an important property
when identifying different types of bacteria with special stains ). Of all the bacteria
known to exist, approximately 4% are known to cause disease in humans.
• An initial step of the identification process is to examine a sample under the microscope so that the bacteria can be classified according to one of three basic shapes,
i) Cocci are three type---> a) staphylococci b) Streptococci c) Diplococci .
Eg: Diseases caused by cocci include streptococcal sore throat, pneumonia, abscesses, food poisoning, gon-
orrhea, and meningitis.
ii) bacilli : Bacilli cause diseases such as botulism, tetanus, diphtheria, tuberculosis, and salmonella food poisoning. E. coli is a
normal bacilli that is present on our skin and in our intestines, but if we come in contact with a specific strain, E. coli O157:H7, it may lead to serious food poisoning, and may even be fatal.
iii) Spirilla: but the infections may be quite serious when they occur. Treponema pallidum is a spirillum that causes syphilis, and cholera is caused by another type of spirillum.
2) Viruses: Viruses are another common type of patho-genic microorganism. They are the smallest infectious agent, and are not really cells. Viruses are either made up of RNA or DNA surrounded by a protein capsule,
and require a host cell to survive and replicate. Viruses cannot be observed using a simple microscope such as is found in most laboratories; they require use of an
electron microscope for visualization. They invade the cells of our body, and use our own structures to make more virus particles. Viruses cause many of the diseases
for which we are vaccinated, including measles, mumps, and chickenpox.
Antiviral medications are available for some viral infections, but because of the way virus particles use human cells, the treatment may
be quite damaging to the host as well as to the virus. Antibiotics are not effective against viruses.
3) Fungi: Fungi are plant-like organisms that flourish in an environment that is dark and damp. Yeast is a type of fungus, and fungal infections on the body are called mycotic infections. Athlete’s foot and ringworm are
examples of fungal infections.
4) Parasites: Parasites are similar to viruses because they require a living host to survive. A genus of bacteria known as Rickettsia is parasitic. Parasites take their nourishment
from the host and require their host cells to reproduce, but they do not utilize the human cell in the same way that a virus does. Malaria is an example of a parasitic disease; the
malaria parasite actually works into the red blood cells of the host and causes the patient to become ill. The word parasite may also be used to refer to larger, multicellular organisms such as tapeworms, which live inside the
human host and use the body to survive.
5) Protozoa: Protozoa are complex single-cell microorganisms, most of which are nonpathogenic. However, there are a few species that can cause very serious infec-
tions in humans. Protozoa live in the soil and water.
