People often use the word germ instead of microbe, so you might think that microbes are all harmful. But some are useful to us.

Useful microbes

Yeast cells are useful to bakers and brewers. Yeast cells can change sugar into carbon dioxide gas and alcohol. This is useful to bakers because the gas helps the bread rise, and it is useful to brewers because it adds the alcohol needed for their drinks.

Bacteria are also useful to us. For example, certain bacteria cause the changes needed in milk to make yogurt and cheese out of it.

Harmful microbes

Many microbes can cause diseases. For example here are some diseases caused by fungi:

• athlete's foot

• thrush

Here are some diseases caused by bacteria:

• tuberculosis, TB (affects the lungs)

• salmonella (causes food poisoning)

• whooping cough (affects the lungs)

Here are some diseases caused by viruses:

• chicken pox (affects skin and nerves)

• common cold

• influenza, flu

• measles (affects skin and lungs)

• mumps (affects salivary glands)

• rubella, german measles

Microbes cause disease when they are able to reproduce in the body. They produce harmful substances called toxins, and damage tissues and organs. We say that someone who has harmful disease-causing microbes in them is infected.