The illnesses are in California, Colorado, Illinois, Indiana, Montana, Nebraska, New Mexico, Oklahoma, Texas and West Virginia. Colorado has the most illnesses with 12 sickened.
The Centers for Disease Control and Prevention said Monday that the listeria was traced to Rocky-Ford brand cantaloupes from Jensen Farms in Holly, Colo. The company recalled the fruit last week after illnesses were linked to its fruit.
There are four confirmed deaths, but officials are investigating two other suspected cases. One death occurred in Colorado, one in Oklahoma and two in New Mexico.
Health officials say the number of people sickened may grow because the incubation period for listeria can be up to a month.
While most healthy adults can consume listeria with no ill effects, it can kill the elderly and those with compromised immune systems. It is also dangerous to pregnant women because it easily passes through to the fetus. In the current outbreak, the median age of those sickened is 81.
Symptoms of listeria include fever and muscle aches, often with other gastrointestinal symptoms.
The Associated Press contributed to this report.