Viruses - are extremely small particles made from protein and either DNA or RNA. They are not made up of cells instead they invade the living cells of other organisms and use them to produce many copies of themselves. Viruses are smaller than bacteria and are enclosed in protective coating. This makes it more difficult to kill them. Antibiotics do not kill viruses. Some diseases caused by viruses include common cold, influenza, AIDS, chickenpox and shingles.
Fungi unlike bacteria may consist of more than one cell. Most of them are invisible to the naked eye, but sometimes they can grow to the size as large enough to be noticed. Many fungi multiply and spread by producing tiny spores that are carried in the air. Sometimes they multiply by splitting themselves in two like bacteria. They thrive in warm and moist areas of the body. They are treated using antifungal medications. Some infection caused by fungi include trush, ringworm and athlete's foot.
Parasites are dependent on the host for their existance. They are multicellular organisms. They can be divided in two groups: ectoparasites (live on human body - on skin or in the hair e.g. scabies, lice, ticks and fleas), endoparasites (live inside of the human body e.g tapeworm, liver fluke).
1.3 An infection occurs when the body is unable to protect itself from micro-organisms such as viruses, bacteria and fungi. Parasites can also enter the body, resulting in infection