The World Health Organization says the A/H1N1 flu has created the fastest-moving pandemic ever. It's nearly impossible to count individual cases.

The agency declared a pandemic on June 11th, but has now it has revised its instructions.

National health authorities need only report clusters of severe cases or unusual clinical patterns.

A/H1N1 can be treated by Tamiflu or Relenza and many patients have mild symptoms. But the WHO warns that countries should remain vigilant for possible mutations of the virus.

It advises testing a limited number of cases to make sure they are in fact A/H1N1.