In situations like this, there is room for argument about how to balance the safety of bystanders against the civil liberties of disease carriers. Even in the case of Mallon, who apparently never accepted the idea that she could feel perfectly healthy but still pass typhoid on to others, the government's ultimate response -- isolation for the rest of her life -- seems excessive, especially compared to the treatment of other recalcitrant carriers.
But at least in dealing with potentially deadly microorganisms that move from person to person, the rationale for government action is to prevent people from harming one another. By contrast, much of what passes for "public health" today is aimed at preventing people from harming themselves.
Activists and politicians use the language of public health to legitimize government efforts to discourage a wide range of risky habits, including smoking, drinking, overeating, under exercising, gambling, driving a car without a seat belt and riding a motorcycle without a helmet. Unlike typhoid fever and tuberculosis, the risks associated with these activities are not imposed on people; they are voluntarily assumed.
In a society that loses sight of that crucial distinction, the government has an open-ended license to meddle in what used to be considered private decisions. Anyone who exposes himself to the risk of disease or injury becomes a menace to public health.