Organisms that cause disease are called pathogens. They are also parasites, living off their host, which does not benefit as a result.
There are 4 types of microbes (micro-organisms) that cause disease;
Protozoa (also known as protoctista).
Viruses are very small, and most can only be seen with an electron microscope.
They just contain a core of nucleic acid (DNA or RNA) surrounded by a covering layer of protein (no cytoplasm, cell membrane or anything else). They are not usually considered as living organisms, as they are very simple and do not carry out most of the living processes which other living organisms perform.
Viruses are not made of cells, and are sometimes referred to as virus particles or virions.
Viruses are inactive when outside of a living cell, but once their nucleic acid is inside they take over the cell's activities. Usually they make many copies of themselves inside the cell, then break out of the cell and infect others. Viruses cannot be killed by antibiotics such as penicillin.
Examples of diseases caused by viruses: flu (influenza), common cold, measles, mumps, german measles (Rubella), smallpox, cowpox, chicken pox, HIV (can lead to AIDS), rabies
Bacteria (the term bacterium is used for single one) are single celled organisms, only visible (as "dots" and "dashes") with a (good) light microscope. They are larger than viruses but smaller than ordinary cells of the human body. They contain DNA but this is not inside a structure like a nucleus. Because of these features, bacteria are placed in a separate kingdom of their own.
Some bacteria produce spores, which can be inactive for a long time and survive exposure to chemicals and heat which kill ordinary bacteria which are growing normally.
Not all bacteria cause disease; some are very useful "in nature". When bacteria infect the body, they can reproduce quite fast. Often they produce chemicals called toxins which have the actual effect which causes an illness. Bacteria can usually be killed by antibiotics such as penicillin.
Examples of diseases caused by bacteria: cholera, tuberculosis (TB), septicaemia ("blood poisoning"), anthrax.
Fungi (singular fungus) are slightly larger than bacteria, and their DNA is contained inside nuclei like plant and animal cells. Nowadays fungi are placed in a separate kingdom of their own. Many fungi consist of thread-like structures called hyphae which branch as they grow and expand to penetrate whatever they are growing on. Others like yeast are made of single oval shaped cells, and they grow by budding off new cells asexually. They are more suited to living in a liquid environment.
Once again, not all fungi cause disease; some are very useful "in nature".
Examples of diseases caused by fungi:
Athlete's foot (not only caught by athletes, or confined to the foot!), ringworm (not a worm), thrush (not a bird)
Protozoa/protoctista are generally single celled organisms with true nuclei and a cell membrane, so they are quite like our body cells.
Examples of diseases caused by protozoa/protoctista:
Malaria, sleeping sickness, dysentery