Hospitals in Hull could be on the brink of a new outbreak of a diarrhoea and sickness bug that forced ward closures in September, health chiefs fear.
Virus victims are being told to contact their GP or NHS Direct
The warning comes as five people with the highly contagious Norovirus went to Hull Royal Infirmary and Castle Hill Hospital on Monday.
Health bosses warned sufferers should report to their GP instead of turning up at hospital.
If they continued to ignore this advice "a new outbreak is possible", they said.
Wards are already facing bed shortages because of a rise in medical admissions.
Dr David Hepburn, medical director for Hull and East Yorkshire Hospitals NHS Trust, said people arriving at A&E with infectious illnesses are adding to the problem.
He said: "We are currently facing an extremely difficult period as the number of people coming to our hospitals is increasing all the time.
"This puts a lot of pressure on our staff who are working incredibly hard to ensure that patients receive the best possible care.
"Our job is made much harder, however, when people ignore the advice about contagious illnesses and continue to present at A&E with sickness and diarrhoea."
People suffering from the Norovirus are considered contagious 48 hours after the symptoms have gone.
Mr Hepburn said it is important patients do not visit the hospital, but call their GP or NHS Direct for advice.