R₀, also known as the basic reproduction number, is a measure of the ability of an infection to reproduce and spread in an unimmunised population. If R₀ is less than one, each infected person infects (on average) less than one secondary person and the infection will die out. If R₀ is greater than one then each infected person infects more than one secondary person and the infection will spread.

R₀ varies greatly between diseases:

From the R₀ figure the proportion of a population that must be immunised to prevent the spread of a disease can be calculated. If we use the population of the UK (61 838 154 according to 2009 figures from the World Bank) then we get the following graph:

You can see that for the most infectious disease on our list, measles, more than 93% (on average) of the population need to be immunised to prevent the disease from spreading (to prevent an epidemic). This is alarming in light of the failure of many parents to immunise their children due to unfounded fears about the MMR vaccine.