Is pre-k worth major risks?

We just had a kid and already people are telling us about daycare and pre-k. My inlaws watch our son while we work (he is 4 months old). We dont want to do daycare because 1.) we dont need it, 2) its extremely expensive and really we cant afford it, 3) we dont want our kid/us to get sick.

I mentioned we might not do pre-k either because in our state it is not free and costs a lot, and again we dont want him/us to get sick.

Also, my husband is immunocompromised, like a cold for me=9 weeks of bronchitis for him. He was hospitalized for over a week when he had the norovirus and on disability for 6 weeks after. We know our son will eventually bring illness home from school but we are trying to prolong it as much as possible. Also it is possible my son has the genetic condition that makes my husband immunocompromised. We will get tests done to see but if so he would also get sick very easily though apparently it is easier to get better from in childhood so it wont be as bad for him until he is an adult.

Anyway, everyone keeps harping on us saying we should def send him to pre-k at least. Do you think it is really worth it, and why? Worth it meaning, draining my 401k to pay for it, and risking my husband's life earlier than necessary.

Our son will go to public school when he is old enough for kindergarten/first grade.