MOSCOW, June 22 (RIA Novosti) - A total of 52,160 cases of human A/H1N1 infection have been registered in 99 countries worldwide, with 231 deaths, the World Health Organization (WHO) said in a report on Monday.

As of Friday a total of 44,287 cases had been registered and 180 people had died.

Mexico has seen the most fatalities, with 7,624 cases and 113 deaths. The U.S. has so far seen 21,449 cases and 87 deaths. Other countries to be hit include Canada (5,710 cases, 13 deaths), Chile (4,315 cases and four deaths) and Britain (2,506 cases, one death).

The WHO decided to raise its pandemic alert level to the maximum of 6 on June 11.

An influenza pandemic is an epidemic of an influenza virus that spreads on a worldwide scale and infects a large proportion of the human population.

There were three flu pandemics in the last century. The 1918 Spanish flu was the most serious pandemic, claiming over 50 million lives. It was followed by Asian Flu in 1957 with up to 4 million deaths, and Hong Kong Flu in 1968 resulting in about 1 million deaths.