The Department of Public Health in Iowa says 245 confirmed, probable or suspected cases of mumps have been reported there since mid-January.
The state has been hit by the worst Mumps outbreak in the U.S. since the late eighties.
According to the Centers for Disease Control and Prevention, it is the nation's only outbreak, and is the same mumps strain as the one that has infected thousands of people in the UK.
Authorities say more than one third of Iowa is affected and the outbreak is considered to be on the scale of an epidemic.
Experts say the outbreak does not appear to be confined to certain age groups or other sectors of the population, although 23% of all the cases are among college students.
Neighbours Illinois, Minnesota and Nebraska are on alert as the infection spreads.
Experts say Mumps is caused by a virus called 'the mumps virus', a highly contagious disease which infects one or more of the salivary glands.
It is spread through direct contact with saliva and discharges from the nose and throat of an infected person, and sneezing and coughing spreads the disease.
People can become infected by just talking to someone who has mumps.
It is characterised by the swelling of the cheeks and jaw, often quite severely, and soreness around the area of the jaw.
Pain in the neck or ear can be one of the first symptoms of mumps but as many as 30% of victims show no symptoms at all.
The illness can take from 12 days to 3 weeks before symptoms appear and is more of a concern in young men because some experience swelling of the testicles which can lead to a reduction in fertility.
Encephalitis and meningitis are also complications but are uncommon.
Treatment involves drinking plenty of water, rest, and medications to control fever.
Infected children should be kept away from school for nine days from when swelling first appear.
There is a vaccine available which protects against mumps.