Having sexual activity/intercourse and mixing bloods. Also if a mother has an STD and has a baby that baby has a chance of getting what she has. Some people don't even know they have an STD so passing it can be by accident but then there are those careless people who don't care which is why you have to be really careful with that stuff!
when they transfer fluids to each other. it can also be passed if they share infected needles. and what kayla said is also right