Bacteria

Bacterial cells tack nuclei. Their cell walls are rigid and composed either of two lipid layers or a single lipid layer. Bacteria form their own genetic material but depend on the host for favourable growth conditions. Different varieties of bacteria thrive either on the skin tissue, which causes for example adolescent pimples on the face, or in the alimentary canal or in the body tissues in the various organs. Bacteria develop resistance to different types of antibiotics by mutation of genetic elements that enter other bacteria and incorporate themselves.

Viruses

All viruses depend upon host cell metabolism for their growth and reproduction, that is, they are intracellular parasites. They are differentiated by their genetic material and by the shape of their protein coat. Viruses cause a major share of all human infections e.g. common cold, herpes simplex, hepatitis, traveller’s diarrhoea, measles, mumps, chicken pox etc.

Fungi

Fungi possess thick cell walls and grow as perfect sexually reproducing forms such as budding yeast cells and slender tubes. Some produce spores. Fungal infections may be superficial e.g. foot rot or may be deep, destroying vital organs such as the lungs or alimentary canal. Many fungi are opportunistic and attack people whose resistance is very low e.g. AIDS patients.

Protozoa

These are parasites, which are single cells endowed with mobility and pliable cell walls. They usually house within the vagina, transmitted by sexual contact or within the alimentary canal spreadby contaminated food and water or within the blood, spread by blood sucking insects like mosquitoes causing malaria, brain fever etc.

How Infectious Agents Cause Disease

Infectious agents establish infection and damage tissues in the following ways:

1. They can contact or enter host cells and directly cause cell death.
2. They may release toxins that kill cells, or release enzymes that degrade tissue or damage blood vessels.
3. They induce host responses that although directed against the invader may cause additional tissue damage leading to pus formation or allergic reactions.