sure why not. the baby has to eventually get those germs in order to jump-start his own immune system so why not from a loving sibling? germs are often the only thing that siblings openly share. i am not suggesting that you take the newborn to visit ill people or kids but viral exposure is really inevitable -- expect it. if parents put kids in day-care at an early age they will most likely be over-exposed to lots of viruses. from lots of different cohorts. in those babies expect colds to come early and often.
can a day-care toddler touch the baby?