system
Given the following post, respond as if you're commenting on Reddit:
human
Why are children especially vulnerable to measles?

Is it because of a general lack of antibodies, or perhaps because the immune system isn't as well-developed?
gpt
In the modern world, almost all adults have been immunized against measles.  In the old days, everyone caught measles as a child and either died from it or became immune, so they didn't get it as adults.  In populations where adults aren't immune (like Native Americans just after Columbus), adults died from measles like crazy.