Once you are infected bacteria grow and reproduce inside the cells of your body. As they grow and divide, many bacteria make toxins (poisons). These toxins can damage your cells. Some bacteria damage your cells directly as they grow. Disease symptoms such as a high temperature, headaches and rashes can be caused by the damage and toxins or by the way your body responds to the damage and toxins produced by the bacteria.
As you have seen, there are pathogens all round us and many different ways they can be spread from one person to another. Your body needs to be able to stop these pathogens from gaining entry. This part of the immune system is called non-specific immunity because it is present all the time and not activated in response to a particular pathogen.
So you think you are clean?
Your skin is possibly the most important barrier to prevent infection entering the body. Cuts and grazes can break this barrier and there are systems to automatically repair any damage. Your skin is a specialised organ that not only protects but also senses the environment and helps to regulate your body temperature.
The outer layer of skin forms a tough barrier to infection that is effective as long as it is intact. Cuts, grazes, burns and hypodermic syringes are all ways that this barrier can be broken and pathogens gain entry into the body. To prevent this happening, the skin can repair itself when any damage occurs.