Health bosses are warning that the capital is particularly at risk of a measles outbreak - as London has the lowest MMR vaccination rate in the country.
Latest figures show an increase in the number of cases in the first three months of this year.
2013 is expected to be the worst year for measles in the UK for the past two decades. Read more about the illness and how it is treated.
If the Government's top adviser on immunisation thinks the risk of an emergency in England is a possibility, I believe him.