Measles completely wipes the immune system of children and leaves them prone to catching other killer bugs, scientists say.
A study of 26 unvaccinated youngsters with measles discovered the life-threatening infection reset their body's internal defence system.
This meant the resistance youngsters had developed against other illnesses were completely forgotten.
Researchers say the bout of measles - which is preventable with the MMR vaccine - reduced their immune systems to that of a newborn baby.
The World Health Organisation decided to revoke Britain's status as a measles-free country. Last year there were 966 cases which was almost four times as many as in 2017
The finding will fuel demands for mandatory vaccinations, as figures show jab rates have fallen around the country.
Public Health England (PHE) says that one in seven children, roughly 500,000, has not been immunised against measles, mumps and rubella.
It comes amid an anti-vaxx movement fuelled by a repeatedly disproven study that claimed the jab caused autism.
Most youngsters who die from measles are killed because of complications triggered by the contagious bug.
Pneumonia affects one in 20 sufferers, while others catch dangerous stomach bugs and one in 10 get ear infections which can lead to permanent deafness.
Roughly one in 1,000 suffers from swelling in the brain, which either kills or leaves patients permanently brain damaged.
Almost all of the 20 areas with the lowest levels of MMR coverage at the age of five were in London, figures have revealed. Only Surrey, which ranked 20th lowest with 79 per cent, was outside of the capital city
The Department of Health and the NHS are expected to produce a comprehensive strategy for improving vaccination rates later this year.
Health authorities said they will:Write to GPs to urge them to promote a catch-up vaccination scheme offering jabs to children and under-25s who missed theirs when they were young.Update information on its websites to address parents' concerns about the safety and effectiveness of vaccines, specifically using scientific evidence to address and disprove misleading claims. Invest more in 'local immunisation coordinators' to promote vaccination in families who are hard to reach and areas with low uptake. Call a meeting of social media companies, such as Facebook and Twitter, to discuss how to stop false information about vaccines spreading online.
For their latest study published in the journal Science Immunology, researchers sought to understand why white blood cells needed to fight infections are wiped out by measles and then come back in force.
The team, which included scientists from the Wellcome Sanger Institute in Cambridge, took blood samples of 26 unvaccinated children who had measles at the start and end of the study.
They found they lost large numbers of a specific type of immune cell they had built up before the disease.
This gave almost two-thirds the immune system of a baby, which has no protection because it has never encountered dangerous bugs.
The researchers say unvaccinated