A few considerations on the measles vaccine: Molecular biology studies reveal that many infectious diseases of humans derive from similar diseases in animals of the distant past. Measles used to be, although in a different form, a disease affecting cows. In places where man lived at close quarters with livestock and where the population density was high such as in cities, measles ‘passed’ to people and became a common disease. Measles so became during the middle ages a disease towards which a large number of people developed a form of resistance towards its more dangerous complications. For this reason, the disease today is serious but usually not devastating. However, in countries where for historical reasons people did not live together with animals and where people did not usually live in cities, measles was much less common but also much more serious since people did not develop a natural resistance.  Towards the year 1500 when the expansion of Europe into the rest of the world was beginning, one of the determining reasons that allowed European armies to overcome much bigger armies and to penetrate enemy lines with almost no opposition is that they brought measles with them. Thanks to measles, that sailors of the old world brought with them, huge armies succumbed. Measles was for Spain, Portugal, England, Italy and Holland an invincible arm that allowed these countries to colonize large parts of the world. Fortunately, today we have a vaccine and no longer use this disease to win wars.