3. Actually, the initial wave of deaths from the pandemic in the first half of 1918 was relatively low. It was in the second wave, from October through December of that year, that the highest death rates were observed. A third wave in spring of 1919 was more lethal than the first but less so than the second. Scientists now believe that the marked increase in deaths in the second wave was caused by conditions that favored the spread of a deadlier strain. People with mild cases stayed home, but those with severe cases were often crowded together in hospitals and camps, increasing transmission of a more lethal form of the virus.