The 1918 Influenza Epidemic was the worst one on record. People infected early in the epidemic had a mild flu, while those later infected had a much...

The 1918 Influenza Epidemic was the worst one on record. People infected early in the epidemic had a mild flu, while those later infected had a much more severe form of the disease. what is the explanation for this observation and what laboratory studies mimic these results? Why do you think this doesnt occur for all infectious diseases?