The effects that an infection has on the body, such as a fever (raised body temperature) or a rash are called the symptoms. Different microorganisms cause different symptoms, but they all damage the body in one way or another. The damage is done to the body's cells.
Some microorganisms damage cells directly. For example, malaria parasites invade red blood cells and multiply inside them, eventually making the cells burst open. Malaria causes flu-like symptoms.
Many infectious microorganisms produce poisons (toxins) that damage cells:
- Some bacteria produce proteins that damage the material holding cells together. This helps the bacteria to invade the body more deeply e.g. Staphylococcus bacteria can cause food poisoning.
- Other microorganisms produce toxins that poison cells, causing fever or inflammation (painful swelling). Some strains of Escherichia coli cause diarrhea by secreting toxic substances.