When Polio went away, so did parental commitment to polio vaccines. Since vaccination has a free-rider component (if everyone else's child is vaccinated, the risk/benefit ratio for vaccinating one's own child may be inverted) this isn't completely irrational. Of course it doesn't work; too many people accept the free ride and the disease returns.
A similar thing is happening with SIDS (A Quiet Revolt Against the Rules on SIDS - New York Times). Disobedient parents giggle over their naughtiness on web sites, telling stories of babies sleeping on their stomachs. So SIDS will return (but infant heads will be rounder).
Willful denial of risk is dumb, but very human. On the other hand, a calculated assumption of a measured risk of infant death is rational, albeit inhuman. We expose our children to significant risks when we drive them to day care, for example. Anyone with a swimming pool in the backyard, or a gun in the house, or a seat on the back of a bicycle is already exposing their child to risks that dwarf the average child's risk of SIDS. We make many compromises in our mortal lives, rationally trading an increased risk of infant death for a night's sleep is by no means extreme. It's just that we usually don't think that way.
What we really need is the 'holy grail' of preventive medicine -- risk adjustment. We need better ways to assign a "SIDS-risk" to an individual child based on birth history, genetics, health status, parental smoking, etc. Then we can place 'sleep on the stomach' into a risk spectrum. For the healthy full term child of a non-smoker with no family history of SIDS and no current respiratory infections the risk of sleeping on the stomach may be comparable to the risks of driving to day care. That is, non-zero, but comparable to other accepted risks. On the other hand for a preterm infant of a smoking mother with a family history of SIDS and a URI it may be comparable to riding on a bicycle seat in heavy traffic.
Maybe children will one day wear a bracelet that signals their risk-adjusted SIDS probability every evening ...