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The hygiene mythbuster

28 November 2016

This issue, Cleaning Matters columnist Seán Derrig reveals the grim truth about norovirus

People associate all sorts of things with winter. For me it’s norovirus and I’m not alone. About a million of us will catch it over an average ‘season’ - which usually reaches its peak about now.

While seldom life-threatening, it’s pretty grim: a couple of days of projectile vomiting and explosive diarrhoea. The real issue with norovirus is once it gets into a confined area such as a school, hospital, cruise ship, hotel or food business it’s really difficult to get rid of. Many assume this is because it’s tough to kill with standard disinfectants – which is true – but that’s not the whole story. Like much in life, norovirus is a numbers game and the odds are stacked in favour of the virus.

For a start the infective dose is very, very low; often fewer than ten virus particles can make you sick. And someone with an infection sheds lots of virus particles in faeces and vomit. How many? About 100 billion per gram of stool. That is a huge number so let’s put it in context. For a start, if you use a disinfectant that kills 99.9% that’s still 100 million organisms per gram left. 

A ten pound note weighs about a gram. So first cut your £10 note in to a million pieces; each will be about the width of a human hair. Now take one of those pieces and cut it into 10,000 more. That’s the size of the microscopic smear of poo that someone needs to leave on their hands after using the loo to pass it on by touching a tap, a door handle or another surface.

And if it’s a food handler the risk of passing the illness on increases enormously because you don’t need ‘secondary transmission’ via a tap etc – they risk introducing it directly into the meals they prepare. This is the principal reason outbreaks in any business or institution preparing food can get out of control so quickly.

So, with an organism that’s prolific, tough to kill and has a low infectious dose, what can you do to stop its spread? Proper hand washing is very effective at removing virus particles so staff being assiduous in their hand hygiene is your best defence, especially after they use the loo. The bad news is the advice that staff are safe to return to work after 48-72 hours free of symptoms isn’t the whole story. If you’ve had norovirus you will still be ‘stool positive’ – shedding virus in your poo – for up to two months after symptoms have eased. Which is why it can keep coming back.