Being a highly industrialized nation, the United States is largely free from most serious communicable diseases found in many developing nations; however, the HIV rate is higher than in Canada and Western Europe, with about a 0.5% infection rate in the overall population.
Two diseases that, while rare, are worth becoming educated about are rabies and Lyme disease. Rabies is more prevalent in eastern regions of the country and may be contracted from animal bites; if you are bitten by any mammal see a doctor quickly—do not wait for symptoms. Lyme disease is spread via the deer tick, which are prevalent in the woodlands and open fields of many rural areas. When venturing into the outdoors, it is a good idea to apply an insect repellent onto exposed skin surfaces that is effective against deer ticks.
Other diseases that are endemic within the United States, but are of far less concern, include Hantaviral Pulmonary Syndrome (found in western regions), Rocky Mountain Spotted Fever (mostly in the Rocky Mountain region), West Nile Virus (all regions)and Eastern/Western Equine Encephalitis (particularly in the mid-west region).
It should be noted that all of the above listed diseases are extraordinarily rare and the medical system of the United States is very much capable of handling any of these when necessary.
For the latest in traveler's health information pertaining to the United States, including advisories and recommendations, visit the Centers for Disease Control and Prevention destination United States website