I do not miss those days.  Once they get older it gets better but it does seem to last forever.  I remember wanting to put them into a full length body-condom the entire time they were here.  Children that age are walking petri dishes.  The good thing is that your BD is developing a killer immune system right now!

One thing - make sure they learn to cover their mouths when they cough or sneeze NOW.  For some reason my skids parents believed it would hurt their feelings to get on them to do it (because they were insane when it came to the slightest whiff of discipline over anything) and they will let it fly even in public.  I kid you not.  Perfectly normal well-dressed middle class young adults suddenly open-mouthed coughing and sneezing at will.  The looks of horror from the people around them are frightening and often people will exclaim indignantly in surprise but the skids don't get it.  They take great offense and believe people are picking on them.  No joke.

Before we got full custody (forced into it due to BM's drug issues) of my 2 SDs, they came home from visits with BM with HEAD LICE EVERY TIME. Luckily, I never got them for some reason, but it would take weeks of cleaning and treating them to get rid of the lice, only to have to do it all over again when they'd go to visit her. 

Once they came to live with us we bleached their hair and dyed it fun colors. No more lice since. 

Daddy should decline visitation if the Skid is infectious.  The primary advantage that an NCP has is the ability to decline visitation.  You nor DH should expose your infant to an actively ill elder non resident sib.

Let BM deal with the sick Skid.