Expecting the unexpected

'Doctors estimate that more than 100,000 people a week are catching the bug.' Wow. That has to be quite an epidemic.

When is an epidemic not an epidemic? When you can see it coming. Lots of diseases, such as the common cold and influenza, are somewhat seasonal but we wouldn't necessarily refer to the peak each year as an epidemic because it is not unexpected based on past patterns. On the other hand sometimes we get a larger peak than normal, such as a new strain of influenza, and it is then appropriate to call it an epidemic. The press doesn't always make this distinction but it is an important one.

There is currently a potential epidemic of norovirus in England. The press seem to prefer to call it the 'vomiting bug' eg. on the BBC website Many wards closed by vomiting bug. However even though you may not have heard of norovirus before it is actually very common, causing about half of all cases of viral gastroenteritis (stomach pain, diarrhea, and vomiting). Viral gastroenteritis is second only to the common cold as a cause of illness in the U.S. The graph at the top of this page shows the seasonal nature of norovirus outbreaks over the past few years. It looks like there was an increase from2001-2006 compared to 1998-2000 but whether the 2008 outbreak will become an epidemic is complicated by the fact that the press reports on it will themselves increase the number of people seeking medical advice. Cases are apparently at a 5-year high with between 100,000 and 200,000 cases a week and an estimated 2 million in total. This raises a couple of interesting points about the definition of an epidemic:

• We can see epidemics on different time scales. There may be an ongoing, longer term epidemic of norovirus since 2001, AND there may be an epidemic this year.
• Epidemics can be confused by publicity. More cases are reported when a disease hits the headlines - this does not always mean there is a real increase in the disease. For example Ronald Reagan's Alzheimer's diagnosis led to a huge press coverage and an apparent outbreak of Alzheimer's in the US. In reality it was almost certainly just due to the reporting and the increased awareness led to improved and earlier diagnosis of existing cases.
  

Norovirus fun true fact: The Norwalk virus was named after Norwalk, Ohio, where an outbreak of acute gastroenteritis occurred among children at an elementary school in November 1968. Noroviruses are named after the Norwalk virus.

Norovirus dubious fact: according to the Daily Mail 'newspaper' in England norovirus 'is 100,000 times more infectious than salmonella.' Hmmm, that is interestingly precise. I wonder what scientific morsel is behind that statement.