Both the human influenza virus known as the Spanish flu and a swine respiratory disease occurred at roughly the same time. The first human cases of Spanish flu appeared in spring of 1918 while the first reports of the swine illness were in the fall of that year.

Some strains of swine flu, including the one that has emerged recently from Mexico, are known to belong to the same subtype - H1N1 - as the Spanish flu. But the classical swine flu virus (an H1N1 subtype of type A influenza virus) wasn't isolated from a pig until 1930, so the connection between the Spanish flu and swine flu hasn't been clear.

One of the reasons the two strains of the virus were not strongly connected was because they had dramatically different impacts.

The Spanish flu, first identified in May 1918 in Spain, was lethal, killing at least 21 million people worldwide. It also was known to induce a lethal infection in a host of other animals, including ferrets, mice and macaques, a primate found in Europe and Asia.

The swine flu that first appeared in 1918, on the other hand, did not have the same impact on pigs, causing only a mild respiratory illness, leaving some to suggest they were not closely related.

2009 Swine flu not as lethal as Spanish flu: U.S. officials

The most recent version of the swine flu also doesn't appear to share the Spanish flu's virulence, according to the U.S. Centers for Disease Control and Prevention (CDC website). The CDC said Friday the new virus isn't as deadly, because it lacks the genes that made the 1918 pandemic strain so lethal.

But to examine the swine flu's origin further, Canadian Food Agency researcher Hana Weingartl and her colleagues tested the resistance of pigs to both the 1918 pandemic virus and the 1930 swine virus. They performed the tests at a biosafety Level 4 laboratory and animal cubicle at the National Centre for Foreign Animal Disease in Winnipeg, where Weingartl works.

They discovered that there wasn't a significant difference in the effects on the pigs between the two viruses, as both caused a mild respiratory disease, mirroring the symptoms first reported in 1918 and 1930.

"These results support the hypothesis that the 1918 human influenza virus and the virus causing the hog flu during the 1918 pandemic were the same," wrote Weingartl and her colleagues.

Weingartl suggests the susceptibility of pigs to the human virus and the timing of the first report of pigs contracting swine flu - in October 1918, five months after the first reported human case - suggest pigs contracted the virus from humans.

"After that, the pigs were likely contributing to the spreading of the virus also back to humans," she said.

Since then, the swine flu virus has changed substantially. The strain that has emerged recently is "quite different" from the original strain of H1N1 virus first identified in Spain in May 1918, said Weingartl.

The CDC in the U.S. said on Friday the new strain is "a very unusual" combination of human genes and genes from swine and avian flu viruses found in North America, Asia and Europe.

The current strain of the virus has killed at least 10 people, nine in Mexico and one in the United States, the World Health Organization said Friday.