However, the report reassured: 'the epidemic is of a mild nature.' By late summer, they were dying. And by autumn, what became known as the Spanish influenza - or the 'Spanish lady' in the United States - became the deadliest recorded pandemic of all time, with 20 to 50 million deaths worldwide.

The good news, says the US Centres for Disease Control and Prevention (CDC), is that this descendant of the 'mother of all pandemics' is much less vicious.

Juergen Richt, a professor at Kansas State University, calls the two 'distant cousins.' There are similarities. For example, the earlier version, like the one that appeared in Mexico last month, was capable of using pigs as hosts, he explained.

Another link, said Professor Lawrence Stanberry at Columbia University, is that both surfaced during the northern hemisphere's spring, later than the usual flu. 'That's curious,' he said.

Perhaps the most startling aspect of the Spanish influenza was that most victims were 20 to 40 years old, the bracket usually least vulnerable to flu deaths.

Data from Mexico continues to be revised and so far, there have been only a handful of confirmed deaths. But reports at the start of the outbreak also suggested a heavy impact on healthy adults, rather than just children and the elderly.

The World Bank says in a worst-case estimate that a pandemic of a similar scale to 1918 would slay 71 million people today, triggering deep global recession.

But despite eerie parallels, some researchers believe the earlier disaster was a one-off. -- AFP