So what does lead do to the body?
The problem for humans and many other animals is that lead is easily absorbed, particularly when we've turned into powders and liquids. It can also be absorbed through meats and vegetable that have already absorbed it, i.e., if those animals and plants were already poisoned by lead. This is known as organic lead, and is more dangerous as it more readily taken up by biological processes.
It can accumulate over time, distributing itself in the body in preference to the regular, beneficial chemicals, causing gradual failure of biological systems, or it can be absorbed in a sudden hit, producing a kind of chemical shock. When cells manufacture proteins, lead replaces the correct metals, distorting the proteins and rendering them useless. In the brain, lead replaces the calcium ions so necessary for generating impulses, causing diminished recall. It's bad enough in adults, but it's a major concern in children's development, as this is a period of increased growth and specialisation. Exposure at this point can result in deformities and defects which likely won't repair. Since lead isn't used in human biological processes, there is no safe level, just avoidance.