Bugs

If it's a nasty infection and I feel poorly I stay away because, in no particular order:

I'm not up to having a noisy, lively DGC around,

I don't want my DGC to catch it or spread it further,

They'd have to stay away from nursery/school which means someone has to take time off to care for them,

Sick children are wearing and I wouldn't want to inflict one on anyone, especially my DC.

Stay in bed Oddoneout!

It's a two-way thing. My GC were very generous with sharing their bugs!

If, however, it's a really bad infection it's probably best to avoid your GS until you're not so very infectious yourself. If I'm unsure I speak to the parents and tell them the situation and ask what they think. Besides if you're feeling poorly you might need to rest.