The 1918 h1n1 influenza pandemic was the most severe pandemic in recent history. h1n1 spread through north america, europe, and asia in three waves during 1918-1919. in the u. s., it was first identified in military personnel in spring 1918. it is estimated that about 500 million people or one-third of the world’s population became infected with this virus. the number of deaths was estimated to be 50-100 million worldwide and 675,000 in the united states. mortality was high in people younger than 5 years old, 20-40 years old, and 65 years and older. the 1918 spread of h1n1 is considered a pandemic. what accounted for the spread of this virus to pandemic proportions? all but one would be a reason. a) public and private sanitation was not as advanced as it is today. b) although vaccinated, the h1n1 was resistant to the drug being used. c) antibiotics had not been developed yet to treat secondary symptoms. d) the flu spread rapidly throughout close living conditions, like in the military and in heavily populated cities.?