You have not provided the complete question here. What are the symptoms of David and Susan that you are referring to and what was the analysis done by them.
As you have mentioned lead glaze, it can be assumed this question is related to lead poisoning. Lead poisoning does not have any immediate effects unless lead is taken as a very large dose. Poisoning over a prolonged period of time, in adults, causes symptoms that include high blood pressure, reduction in mental functioning, memory loss, infertility in men, and miscarriages in women.
Lead glaze is often used in pottery as it very attractive to look at. If the pots are baked at high temperatures and the glaze properly formulated the chances of lead leaching into food stored in them is almost zero. On the other hand, when glazing is not done properly, it can lead to lead poisoning. How much lead leaches into the foods depends on the type of food stored, how often the pots the heated and if there is high acidity.