The Great Influenza

While the spring wave of the influenza pandemic wasn’t especially lethal, the second wave was. Several explanations have been proposed: the first is that the deadly disease was caused by a different virus than the mild disease. But this is unlikely, since victims of the first wave had significant resistance during the second.

The second possible explanation is that while in Europe, the American-originating virus encountered a European variant, and a new variant arose when the two viruses attacked the same cell and “reassorted” their genes in the process to create a deadlier version. Most experts today, however, also believe that this possibility is unlikely.

The third, and most likely, explanation is that the virus adapted to humans. In 1872, a French scientist studying anthrax in rabbits discovered that as the virus passed through different rabbit victims, it became more efficient at killing newly infected rabbits. The flip side of this, however, is that if a virus becomes too efficient, it runs out of hosts and destroys itself, so most of the time virulence will stabilize and eventually recede. It seems like this pattern of increasing virulence is similar to what happened during the 1918 influenza outbreak.

On June 30, 1918, a British freighter ship docked at Philadelphia and was carrying a deadly disease. Examining the British crew at the University of Pennsylvania Hospital, the physicians and med students concluded that the crewmembers were dying of pneumonia but had strange influenza-like symptoms. The rumor was that these crewmembers died of Spanish influenza. The disease, however, didn’t spread.

Most history of the 1918 pandemic portrays it as abrupt and unexpected. In fact, however, the deadly second wave happened more gradually. Deadly outbreaks in the earlier part of the year remained limited in scope, like at one French base where five percent of all recruits died. By the end of August, the lethal variant of the virus had established itself in three continents—Europe, Africa, and North America—and it was poised to break out.

First, there were signs of an outbreak at Brest, a French city where many American soldiers landed. Soon after, influenza spread through Freetown, Sierra Leone, which was a major center for coal, killing at least three percent of the Africans living there—or perhaps even twice that. Finally, an overcrowded ship arrived in Boston. Two sailors were sick with influenza, but the number quickly multiplied, and so did the death toll.