Officials at York Hospital have written to nearly 600 patients after a member of their healthcare staff was diagnosed with tuberculosis.
The member of staff had worked on ward 16 at York Hospital
The unnamed woman has been ill since last September, but the disease has only just been diagnosed.
Managers at the hospital said the member of staff was deployed on ward 16, a surgical unit, but stopped work in March when her coughing got worse.
There are no notified cases of the disease being passed on in the region.
However, hospital officials said the woman had not recently travelled outside the UK.
Symptoms of tuberculosis
Sense of tiredness
About 500 patients who have spent less than two weeks in the ward since September will receive letters in the post on Thursday morning.
The letters reassure patients there is a very low risk that they have contracted the disease but advise anyone worried about the situation to contact their own GP.
A further 80 people - who had spent two weeks or more on the ward - have had separate letters advising them to go for a chest x-ray.
Both letters say: "Please reassure any friends and relatives who may have visited you while you were on the ward that they are not at risk and do not require any investigations whatsoever."
A helpline has been set up with the help of NHS Direct and the Health Protection Agency has been informed.
Staff were told on Wednesday and they are being dealt with on the same basis as the patients.