Microorganisms are tiny living organisms, usually made from only one cell. Some are quite useful, such as yeast that is used in baking, but most are quite dangerous, as they can cause disease. Microorganisms that cause disease are called pathogens. There are four types of pathogens:
- bacteria, examples of diseases they cause are cholera, tuberculosis, salmonella, tetanus and pneumonia
- viruses that cause AIDS, measles, rubella, warts and flu
- fungi that cause athlete's foot, thrush and even throat infections
- protozoa that cause dysentery and malaria
Diseases that can spread from person to person are called contagious. Vector animals, like the mosquito and the house fly carry microorganisms that cause a disease but they do not suffer from the disease themselves. An example of this is malaria, which is caused by the protozoan Plasmodium falciparum, which is transmitted to humans when they are bitten by female mosquitoes in some tropical areas. This parasite feeds on red blood cells from humans causing fever which can often be fatal.
Fortunately, our body can fight pathogens with the help of white blood cells. This is active immunity, as our body actively tries to fight disease. Medical research has developed antibiotics and vaccines to save even more lives. This is called passive immunity.