1908
You made me curious. Bubonic Plague wasn't a widespread threat in Europe in the 20th century, though there was a nasty outbreak of cholera and typhoid fever not long before...
But no: a search with "+1908 +london" came up with the Olympics. Paris hosted in 1900. In 1904, they were held in St. Louis, from July until Thanksgiving, while the World's Fair was going on there (April 30 - December 1).
