Mersa

I will have to disagree with the other posters. MRSA isn't actually that contagious or infective, at least not any more than regular staph. Plenty of people carry it on their skin just like regular Staph and it only causes infections if it gets into an open wound. The problem with MRSA is that it is very hard to treat because it is resistant against many common antibiotics, hence the name Methicillin Resistant Staphylococcus Aureus (MRSA).
It used to be restricted to hospitals but it increasingly common in the community.

You can easily protect against MRSA by washing your hands with regular soap and water and covering any open wounds and by not sharing things like razor blades and towels.

MRSA infections can be serious and lead to hospitalization, but most are simply a little pimple or boil that is slow to heal and requires topical or oral antibiotics to go away.

It's spelled MRSA so when you google this, be sure to use the MRSA (it's an acronym for Methicillin-resistant Staphylococcus aureus, basically it's a staph infection that's resistant to the usual medications used to treat staph) and you'll get the accurate info that you need. I would think it depends on when the boy had it and how long ago it cleared. Also...where does a child that young pick up MRSA? It's usually something one contracts in a hospital so if this child is frequently in and out of the hospital for other medical issues that might be something to consider, but if he had it in the past and his docs confirm that he no longer has it, I don't think you have anything to worry about but some quick internet research or a call to your pediatrician should give you a clear answer.

It's actually MRSA, some people pronounce it as mersa. It depends on the infection and how it is being contained and treated. How the environment is being treated. It is crazy infectious, and some patients in hospitals actually get quarantined to a degree due to the severity of the infection.

If it is just a skin boil, which is more common among healthy people, both young and old... the child needs to have it covered at all times and if the home is totally disinfected, than short visits should usually be fine. But keep physical contact minimal, and it shouldn't be that infectious.

My bil's (who is a med student) young daughter got a little MRSA spot on her cheek and it cleared up quickly, it was treated promptly and seriously.

My mil has a large MRSA boil and it won't go away, even though it's been on many tough meds. She keeps it covered, cleaned and medicated when around us. But she still struggles with it.

My own father died of MRSA, however he was already sick and it entered his bloodstream through an iv in the hospital, that is the deadliest form.

So, there are many types of strains and levels of seriousness. A small boil, if taken care of and covered, shouldn't be too much of an issue if you are all being sanitary as well..
Some good info for you:

http://www.webmd.com/skin-problems-and-treatments/underst...

http://www.mayoclinic.com/health/mrsa/DS00735

My husband is a MRSA carrier, so we are well versed in this. :) It rarely results in something serious like a hospital stay or anything like that. There are drugs to treat it if you get it (which you can contract from anywhere, really). It is safe to be around the child unless the child has open wounds that have tested positive for mrsa or staph. Know what to look for staph infections, and treat promptly if you notice signs. That's my best advice!

MRSA (and staph) are naturally occurring on our skin. However, when it gets into an open wound, it spreads and is highly contagious.

I personally would not be anywhere near that family.