"Has lead been used a lot in water lines in the past?"
Yes. Even the word "plumber" is a derivative from the word lead. We understand that Chicago required lead water service line until about 1987 (if true, so much for code and local traditions rather than health concerns). The State of California had one of the first strong anti lead campaigns. Now lead is not allowed in any water service anywhere in the U.S.. If you see a lead water pipe we highly recommend replacing it. If it isn't yours we recommend reporting that to your local health officials.

"Isn't lead dangerous?"
Using lead products for drinking water is against all U.S. codes and not advised. Using lead for flashings is hard to beat as lead is the most durable flashing that we know of (although some areas now do not want you to use it due to children playing with it as well as a small potential is there to contaminate our environment with too much lead). You should wash your hands after using lead products and many recommend using gloves. Lead isn't like mercury and it won't go into your body just by holding it but if a child nibbles on it or you place your hands in your mouth after touching a lead product you could ingest some lead. We don't recommend using lead products for anything.