 In the economic sense of the term, public goods are non-rivalrous, which means that one person’s enjoyment or consumption of the good does not reduce the ability of others to enjoy that good. Public goods are also non-excludable, which means it is very difficult to prevent others from enjoying the public good. (See the entry on  the free-rider problem.)  Paradigmatic public goods in public health include herd immunity, sanitation, and clean air. Not all the goods of public health are public goods in this sense, however.