My brother spent the first 10 years of his life in and out of hospital. He looked the same as any other kid, wanted to go to birthday parties and play with other kids, but one bad cold/ infection could put him in hospital for days as his body could not cope, many times he had grand mal seizures as a result, many times he was in intensive care. Exposure to anything worse (such as measles etc) would have possibly been deadly. My poor Mum was always torn, she wanted to give him a relatively normal life and let him do the things other kids do, but there was always one kid sent to school sick because the parents couldn't afford a day off etc....this may have a monetary effect on these parents but for my brother it could mean a lot worse. He was also unable to have any vaccinations as his body could not cope, so he relied on others around him protecting his vulnerable immune system.
So, when i read the posts by the people who send their kids to daycare / school sick as you "can't afford" to keep them at home, i shake my head...If you say it isn't a selfish decision, well actually, it is.