"Underlying conditions" have been held responsible for many severe cases of swine flu in New York City. These conditions include diabetes, asthma, heart disease, lung disease, weakened immune systems, obesity and other conditions that aggravate symptoms. The term has been used to cover all manner of things since swine flu has become a presence in New York.
Originally used to reassure people who did not have any of these "underlying conditions," the term could potentially scare much of the population, as the conditions are fairly widespread. About 50 percent of swine flu hospitalizations globally and about 70 percent of those in the United States seem to involve underlying conditions.