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.