Do newborns have a greater immunity than, say, a 6 month old?

I don’t think their immune system is developed until they’re six months old. One of the perks of breast feeding is that the baby gets antibodies from the mother.

Also, while gross, I don’t think kids get sick from any of the activities mentioned. (Unless they’re eating old paint chips. But you can’t build an immunity to lead poisoning.)

Newborns are at high risk for colds or other infections for the first 4 to 6 weeks of life. That’s because their immune system is functionally immature. Babies do get some immune protection from the antibodies they receive from the placenta before birth and through the mother’s breast milk. But there are many germs that they are not protected against. A virus that causes a mild illness in an older child or an adult can cause a more serious illness in a newborn.
Young kids fight a host of ongoing viruses and bacteria as their immune systems continue to mature and strengthen.

If I gave birth to a baby in January or February in the height of cold and flu season; for the first 6 weeks I would not let people touch it without washing their hands well first. I would try to keep the baby away from children. Children are germ vectors, they are sick constantly. Of course if it is a second or third child it is not practical for the baby to not be touched by their sibling, but you can try to have everyone have washed hands before touching the baby initially and then relax the rules later.

As far as dirt, the only risk really might be a tummy infection if there is ecoli on the floor from using outside shoes indoors. This is part of the reason shoes should be removed at the door. But, most traces of “dirt” in the house isn’t going to harm anyone. Germs like flu, colds, sinus infection, is a whole different thing. Bacteria in dirt are things like tetanus and anthrax, and very rare, and special circumstances.