Most americans born before 1970 have a permanent scar on their shoulders from the smallpox vaccine. why don't younger americans have one?

I believe the answer is: Smallpox has been eradicated by a worldwide vaccination program

Vaccines develop an immunity toward the virus. In order to survive, virus need to obtain a host so they could replicate. Due to the vaccine, the viruses cannot find these hosts. Because of this, smallpox virus started to gradually decreased and eventually eradicated.

the first settlements began along the huang river because of the importance of the huang river in the beginning of china. it goes back to the story, “the tale of emperor yu”, which references the tale of the great flood, which was caused by the huang river and was believed to create and influence china’s making.