I saw this image earlier today, and was curious about the data it contatins. https://upload.wikimedia.org/wikipedia/commons/e/ed/Measles_US_1944-2007_inset.png

1. Why did meseales infection rates vary so much year to year? Is this normal for infections diseases?

2. Why were two vaccines needed? Did they always anticipate needing two vaccines?

3. How lethal was meseales in the past? Did its lethality change over time?

4. Why did the government start collecting data in 1944?

Thank you for your time.