1. Cylindrical (rods)
2. Cocci in Clusters (e.g. gram-positive staphylococci)
2. Spiral-shaped (e.g. spirilla, heliobacter pylori)

In view of the huge number of different bacteria inhabiting the human body, only very few species cause diseases.

Bacteria of the normal flora only cause infections if the host’s immune system is impaired or if they gain access to other parts of the body. These kinds of infections are termed endogenous or opportunistic infections. The invasion of a bacteria from the outside via the skin or mucous membranes is called an exogenous infection. Certain bacteria, being particularly hazardous and virulent, will always, after invasion, cause diseases in humans. Most infections result from bacteria's tissue invasion, adherence and multiplication in the host, initiating an acute inflammatory reaction. Only a few diseases are caused by toxins produced by bacteria outside the host, e.g. staphylococcal food poisoning.

1. Virus (cubic) (e.g. hepatitis)
2. Fungus (e.g.candida)
3. Virus (helix) (e.g. influenza)

Viruses are the smallest of all parasites. They need a host cell to multiply because they do not have a metabolism of their own. To reproduce, they use humans, animals, or even bacteria as hosts. The basic infectious material penetrating the cell is the nucleic core (RNA or DNA) that usually initiates the infection on its own. Several hundred different viruses can cause relatively harmless infections such as the common cold, but also life-threatening diseases like hepatitis or AIDS. A virus infection may weaken the host, increasing the risk of secondary bacterial infections.

Fungi can cause allergic reactions or infections, affecting the lung, skin and genitourinary tract; sometimes resulting in systemic diseases. Most fungi are not usually pathogenic unless they enter a compromised host. Persons under treatment and conditions impairing immune responses are particularly prone to opportunistic fungal infections. Antibiotic therapy can disturb the normal vaginal flora, increasing susceptibility to vaginal yeast infection.