Systematic pediatric evidence shows that the morbidity rates for children in day care are increasing in the group size.Sick children are usually cared for at home by parents. This creates a negative externality of parents' labor force participation. The social optimum implies lower group size than the non--intervention market equilibrium. We study the optimal tax policy. The cost of labor force participation should be increased. This can be done by either or both a tax on day care services and a home care allowance. The cost of providing day care should be decreased by a subsidy to entrepreneurs running day care centers. This policy will decrease the group size. It is, however, not necessarily the case that this will decrease labor force participation.