Anyone who’s spent five minutes with a toddler knows that kids can be a grimy bunch. This hour, we’ll talk about how exposure to some germs is actually good for a child’s health with Brett Finlay, a microbiologist specializing in bacterial infections at the University of British Columbia. He writes about the idea in “Let Them Eat Dirt: Saving Your Child from an Oversanitized World” (Algonquin Books).