The value of LD50 for a substance is the dose required to kill half the members of a tested population after a specified test duration
This is, effectively, how oral flee/tick medications work in animals. It basically turns the animal’s blood into a mild poison. The poison is mild enough that it doesn’t affect the animal but it’s enough to kill small insects like flees, ticks, or mosquitoes.
Social Security Numbers were never intended to be used the way they are. They were expressly for identifying you to the Feds for the purpose of SSI. Business just looked at that and said “hey, that’s a good way to identify people” and ignored the feds when they asked politely for them to fucking not.