The cause of the norovirus outbreak at Dunedin Hospital will probably never be known, a virology expert and a Public Health South medical officer say.
Staff and patients continue to fall ill, with two more patients and three more staff contracting the highly infectious virus since Thursday evening.
Almost 200 staff and patients have caught the virus since the first outbreak on August 4.
Associate Prof Vernon Ward, from the University of Otago's microbiology department, said the patchy outbreaks across multiple wards suggested there was not a single source of infection.
It was not likely the infection was being spread through hospital food, as he would have expected many more cases if that was the cause.
Norovirus was common in the community and the original infection, or infections, would have been brought into the hospital, he said.
Medical officer John Holmes said investigating an outbreak involving even 10 people could be challenging when trying to find a common cause of infection.
Investigating an outbreak with multiple sources which had been running for three weeks was unlikely to yield definitive answers, Dr Holmes said.
Dunedin was not the only hospital to battle the virus.
A norovirus outbreak at Princess Margaret Hospital in Christchurch last year took six weeks to overcome.
Dunedin Hospital staff had sought help from other hospitals which had dealt with major outbreaks, but none had yet offered any new ideas or suggestions not already in place at Dunedin, Dr Holmes said.
Dr Holmes stressed the importance of vigorous hand washing with soap and water, along with thorough drying, to prevent the spread of the virus.
If hands could not be washed with soap and water, wiping with alcohol wipes would reduce any viral load on them.
The virus is spread by direct contact and can be picked up by touching a surface an infected person has touched, or through contaminated food.
Hands should be washed after going to the toilet, before preparing food and before eating food.
People should not put their hands or fingers in or near their mouth.
Presbyterian Support Otago chief executive Gillian Bremner said some of her staff had become ill, with vomiting and diarrhoea, but testing had not yet confirmed if it was norovirus.
Staff had been sent an email reminding them how infectious the virus was and what precautionary steps they should be taking.
