Hundreds of hospital staff are being vaccinated against measles after six nurses contracted the disease.
The outbreak at the Central Middlesex Hospital in north-west London began after two children were admitted with measles last week.
The nurses were treated in an isolation unit at nearby Northwick Park Hospital, but have since been allowed home.
Hospital visitors from the past few weeks who are feeling feverish are advised to contact their GP.
A hospital spokesman said: "Two children had measles and it seems the staff caught it from them.
"The trust is working with the Health Protection Agency to take precautions and ensure we don't get any further cases."
It comes as it is announced that a 13-year-old boy in the North West has died from the disease.
The illness, which spreads easily through contact, poses a serious risk to children, pregnant women and those with weak immune systems.