Something fishy: How to eat seafood safely

It's an important lunch date. You order a crisp glass of champagne renowned "for its fresh minerality" (or so says the menu), and set about tackling a mountain of oysters. A deal is struck between you and your business associate. But 24 hours later, it is your digestion that writes the ultimate cheque.

The Fat Duck suffered a near-fatal illness in March when a dodgy batch of oysters were infected by the rare norovirus. It made 529 of the renowned Bray restaurant's customers sick and closed it for near on three months (its relationships may take longer to rebuild). So where does that leave the zealous scooper and swallower of shellfish? Is it safe yet to venture back into your local palais de moules marinieres?

Well, according to Robin Hancock, the proprietor of Wright Brothers, an oyster wholesaler which supplies top restaurants including Scott's, The Ivy, Le Caprice, The Wolseley and Mark Hix's Oyster and Chop House (they also have a stall and restaurant in London's Borough Market and in 2005 took over the Duchy Oyster Farm on Cornwall's Helford River, where oysters have been grown since Roman times) the risks are overblown. In fact, he says, oysters should be enjoyed because they are full of vitamins, iron, calcium and are low in cholesterol.

"I would like to set the record straight," he says. "Food poisoning from oysters is something from the past. We sell four to five tonnes of oysters a week – that's nearly 60,000 or 2.5 million a year – and we get maybe four or five cases of food poising in that time. What happened at the Fat Duck was somewhat of a freakish occurrence." Several thousand fishermen breathe a sigh of relief.

If you are particularly worried about catching something, there are a few simple steps you can follow when buying oysters or eating out. "When you buy oysters, mussels and clams you need to make sure the shell is completely closed," says Agnar Sverrisson, the Raymond-Blanc-trained head chef at the Mayfair restaurant and champagne bar Texture. "It needs to be quite heavy and difficult to open; the harder it is to prise apart its two sides, the less likely anything bad can get in to infect it. If it is full of juice, alarm bells should be ringing. The key is also to open it and eat it straight away. After an hour or two it might get contaminated. It's very important; here we ask people to open them as they go." Hancock recommends the old adage of "checking to see if the toilets are clean" when venturing into a restaurant; general levels of hygiene can be a useful clue. There will always be a risk if you are eating raw oysters; if you are really worried,order something cooked.

At this point it might be useful to understand the background to oyster contamination (not that it is visible with the naked eye). In the UK, local authorities classify the water in which oysters grow from A to D (A being the cleanest, D being the least suitable for consumption). Oysters from A you could hypothetically remove and immediately consume without a problem. Anything below B needs to be purified (and D is totally unsuitable). For the purification process, known as depuration, the oysters are suspended in water which has been treated with ultraviolet light (this generally kills the majority of dangerous microbes). As a mature oyster will filter five litres of water an hour, the oysters are left for 42 hours; the ultraviolet-light-treated water passes through the oysters' innards and kills anything that might cause you, or them, trouble.

Unfortunately for us, and the customers at Bray, it doesn't apply to all potential nasties. "European legislation requires local health authorities to test for E.coli and salmonella but not viruses," explains Hancock. Viruses are often found within the flesh of the animal and are never excreted; this makes the depuration process less effective. "Norovirus is a disease spread when people defecate. It is possible, if the oyster grew in an area where there is an archaic sewage system, that if there is a lot of rainfall the sewage might pass into the areas where shellfish are growing." There will always be a risk of infection, though Hancock calls Bray "a one off" incident. While the press has criticised the restaurant's practices of not sending ill employees home, it could have done nothing to stop the arrival of infected oysters.

Thankfully, if you're that worried about eating raw, there are some tasty alternatives. "What we do is our signature dish of moules mariniere; steam the oyster for a few minutes in butter, wine, shallots, a bit of parsley, you can't go wrong, though I will always recommend eating them raw on the half shell," concludes Hancock. "We should not make too much of the viral thing; it is exceptionally rare. Again, I think the staff at the Fat Duck – where they are obsessed with a clinical, almost scientific preparation of food and are more than aware of these processes – were incredibly unlucky."

Tips for home cooks: Shellfish safety

Choosing Buy seafood from a reputable source and make sure it reaches your fridge quickly.

Chilling Keep in the fridge and be sure to keep covered or wrapped. Don't store shellfish in water.

Freezing For up to three months. Thaw slowly in the fridge or in the microwave.

Cooking Discard oysters, mussels and clams if shells are damaged or not closed.