For me it would depend on how sick he was. When my DS had conjunctivitis he was still very happy and not too bothered by it, so I was happy to have my mum watch him. My parents are usually happy to have him but if they're busy all my other friends who I would trust to have him, have their own little ones so I wouldn't want to make their kids sick too.
Recently he had HFM, he was so sick in the first 2 days with awful fevers, wouldn't eat and just wanted to cuddle all day. I wasn't working those days anyway but if it had been a work day I would have called in sick as he obviously just wanted me.