Diet and nutrition-
The main food groups are carbohydrates, proteins or fats.
The metabolic rate increases as we exercise and stays high for a while afterwards. People who exercise regularly are usually healthier than people who don't.
Defending against infection-
Pathogens are microorganisms - such as bacteria and viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens by producing antibodies that destroy the infectious microorganisms, and antitoxins to neutralise toxins created as a by-product.
Bacteria and viruses are the main types of pathogen. Bacteria are microscopic organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - that's 10 millionths of a metre.
Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill.
The nervous system allows the body to respond to changes in the environment in a…