Mexico has introduced strict safety measures to combat a new flu strain that has killed 20 people and is suspected of claiming up to 40 others.With up to a thousand people now said to be infected, authorities have been urging people of all ages to wear face masks. A vaccination programme is underway but the emergence of a mixture of viruses has taken experts by surprise and forced Mexico’s President to appeal for calm. “We are taking all the necessary measures to confront the problem, which we found out about yesterday after international health authorities and the most prestigious laboratories had a chance to examine it and determine its nature,” said President Felipe Calderon. Washington is also monitoring the situation after at least eight people developed symptoms in two US states but recovered. Mexico has shut schools, museums and called off public events. However, global health officials have stopped short of declaring a pandemic. The original swine fever is rarely transmitted to humans. Health experts believe the flu is a new, mixed strain mutated from a respiratory disease found in pigs.