What is meant by disease?
We will return to Spain later but first it is necessary to define the various terms that will be used. By disease, I mean communicable or infectious diseases i.e. those caused by infection with microorganisms (viruses, bacteria and parasites) that can pass directly or indirectly from one human to another. The outcome of any infection depends on many factors including the ease of transmission, the rate of multiplication of the microorganism, and the ability of the host to mount an immune response. Infectious diseases play important roles in natural selection and our ancestors, as they spread throughout the world, experienced new infections to each of which a proportion of the population was particularly susceptible and died while the others became infected, recovered and were immune on subsequent exposure to the same infection. Because the most susceptible individuals die they cannot contribute their genes to the next generation and thus, over time, the population as a whole becomes inc