The stomach bug affecting hospitals and rest homes around the country has struck again. 

About 30 residents of a Kapiti Coast rest home have come down with the bug, a type of norovirus which can cause explosive diarrhea and projectile vomiting. 

The bug is also extremely resilient. It can survive on carpet for weeks and still remain infectious. 

In the past week the norovirus outbreak has led to operations being delayed and wards being isolated in Wellington. 

In the Waikato, it struck three rest homes this month, as well as an industrial workplace and three rest homes last month, taking the number of people infected to 180. 

It has affected about 80 per cent of the people at the Waikato sites it has infected. 

An outbreak at an Auckland City Hospital older people's ward last month affected five people. 

Hospital spokeswoman Megan Richards said this week the outbreak was contained by isolating the patients and implementing other infection-control procedures. 

"We have one or two outbreaks of norovirus per year usually, mostly in winter." 

In this year's first quarter, eight outbreaks were reported in the Auckland region. Of the six in rest homes, one became the source of the Auckland City Hospital outbreak. 

An Auckland Regional Public Health Service physician Dr Greg Simmons said these figures were similar to previous years. 

In 2002, the Institute of Environmental Science and Research recorded 26 outbreaks and 849 cases of the illness, including two deaths, in elderly care facilities and hospitals. 

The norovirus group of viruses can be picked up and ingested from objects, infected people, or in the air from "aerosolised" vomit. 

A Public Health Service report says they can survive outside the body for more than a week. 

"One norovirus outbreak among carpet layers was thought to have been due to occupational exposure while removing carpet at a hospital some 13 days since the last case there." 

Dr Simmons said people should make sure they wash and dry their hands thoroughly if someone in the household was suspected of having a norovirus stomach upset. 

The sick person should be isolated. One person should be designated to provide care and that person should not prepare household food. 

Bathrooms, toilets, door-knobs and other affected surfaces should be cleaned with chlorine bleach in a ratio between 50 mls/10 litres of water to 200 mls/10 litres (based on supermarket bleaches that range from 2-5 per cent concentration). 

"With soft furnishings, all you can do is wash with warm soapy water, but ultraviolet light is good [for disinfection], so put them outside. 

"Norovirus can be difficult to control and can get away in rest homes ... Left to their own devices these outbreaks can go on for months if your infection control isn't good." 

Norovirus tips 

* Wash hands thoroughly 

* Isolate people who come down with the bug 

* Clean bathrooms, toilets, door-knobs and other affected surfaces with chlorine bleach