What does your immune system guard you against?
Invading germs are usually bacteria or viruses. Colds, flu, mumps, German measles and chicken pox are all caused by viruses: tiny particles that enter healthy cells. Bacteria are much larger than viruses, but still much smaller than a human cell. Most bacteria are harmless, or even helpful. The billions inside your gut help to digest vegetables and also to make some vitamins. But some types of bacteria are harmful, including those that cause stomach upsets, tonsillitis, ear infections and meningitis.