So by pathogens, we mean some type of agent that can infect the body as well as the fetus and basically cause harm to that developing fetus. So we have many different types of examples. For example, HIV. So if a mother has HIV and she becomes pregnant, then that baby will also have HIV. And that's because that virus is small enough to actually cross along that placental membrane. Another type of dangerous virus that can affect that fetus is rubella.