My three grandchildren were raised in a house built in 1924 from birth until the ages of 5,4,2. (Pittsburgh, PA.)The two oldest had gone to daily childcare centers. The youngest was able to stay home with Mom as she quit her job and became a SAHM. He was checked out thoroughly after failure to gain weight. You guessed it--lead poisoning. They don't know how he got it--he wasn't eating the paint or anything like that, and in fact, the place had been painted over with lead-free paint in later years. There was a big involvement by the health dept., etc. etc. (This was a very nice house in a good neighborhood, btw.) DD and s-i-l spent a lot of money on re-painting the interior, but also had to hire a contractor to scrape away lead-infused soil in the yard. Expensive and very puzzling--they think it did not effect the two older kids because they spent so much time out of the house. The "little guy" is still on the small size of normal to this day. Scary stuff.
After that, I had my house in another state (New York) tested for lead because it was built in 1938 and the grandchildren visited there. Sure enough, lead levels were high enough to need remediation before I could sell the house. Pain in the neck and expensive, but you do what you gotta do.