At least 12 people in seven states have died after eating cantaloupe contaminated with listeria, in the deadliest outbreak of food-borne illness in the United States in more than a decade, according to public health officials.

Many of the deaths involved elderly people, who are especially susceptible to the aggressive pathogen.

The cantaloupes were grown by a Colorado company, Jensen Farms, which issued a recall earlier this month. The melons, a type marketed as Rocky Ford cantaloupes, named after a region in Colorado, were sold around the country.

The federal Centers for Disease Control and Prevention reported last week that there had been 55 illnesses and eight deaths in the outbreak. Four people died in New Mexico, two in Colorado and one each in Maryland and Oklahoma, according to the C.D.C.

But the numbers have increased. On Monday, a state health official in Texas said that two people had died from the strain of the bacteria there. Officials in Kansas and Nebraska said Monday that lab tests showed that a death in each of those states was linked to the outbreak, bringing the death toll reported by state and federal authorities to at least 12.

The number could continue to rise as investigators wait for lab results that could confirm whether several other deaths were related to the outbreak.

In addition, the Missouri Department of Health said Monday that an elderly person in that state died after being infected with the bacteria but that the infection was not considered the primary cause of death. It was not clear whether that death would ultimately be counted as part of the outbreak.

The C.D.C. was expected to provide an updated count later on Tuesday.

Officials said that most of those who died were over age 60. At least two were in their 90s.

Listeria is a common but dangerous bacteria that can cause severe illness, especially among the elderly, the very young and people with compromised immune systems. The pathogen can also cause pregnant women to have miscarriages.

John N. Sofos, a professor of food safety at Colorado State University, said that many people who are infected might have only mild symptoms, such as diarrhea. But in others, especially those in the most vulnerable categories, the bacteria can aggressively move out of the gastrointestinal tract and attack muscle tissue or the spinal cord, leading to much more severe illness such as meningitis.

For that reason, the death rate in listeria outbreaks is often much higher than with other forms of food-borne bacteria.

William Marler, a Seattle lawyer who represents victims of food-borne illness, said this outbreak may turn out to be especially deadly simply because cantaloupe is a food eaten by many older people.

“Sometimes in outbreaks, it’s the population that’s consuming the food that drives the numbers,” Mr. Marler said. “In this instance, you’ve got a lot of people 60 and older who are consuming cantaloupe.”

The outbreak appeared to be the third worst attributed to any form of food-borne illness, in terms of the number of deaths, since the C.D.C. began regularly tracking such outbreaks in the early 1970s.

The deadliest outbreak in the United States since then occurred in 1985, when a wave of listeria illness, linked to Mexican-style fresh cheese, swept through California. A C.D.C. database says that 52 deaths were attributed to the outbreak, but news reports at the time put the number as high as 84.

The second deadliest outbreak was in 1998 and 1999, when there were 14 deaths in a listeria outbreak linked to hot dogs and delicatessen meats.

With Tuesday’s updated death toll, the Rocky Ford cantaloupe outbreak surpassed the 2008 deaths associated with salmonella-tainted peanuts and peanut butter produced by a Georgia company, the Peanut Corporation of America. That outbreak, which drew a large amount of news coverage, killed nine people and sickened more than 700.

The huge outbreak this year in Europe of a rare form of E. coli bacteria attributed to fenugreek sprouts killed at least 50 people.

Listeria is a common bacteria that can be found in soil, water, decaying plant matter and manure. A strain of the organism, called Listeria monocytogenes, was first found to cause illness linked to food in the early 1980s. Since then, only a handful of listeria outbreaks have been associated with fresh fruits and vegetables. The majority of outbreaks were caused by tainted meat or dairy products.

It can take more than two months for a person exposed to the bacteria to fall ill, which means that it is often difficult to identify a food that carried the pathogen.

Unlike some other bacteria, listeria also grows well at low temperatures, meaning it can be difficult to eliminate from refrigerated areas used to process or store foods.

The Food and Drug Administration said it has found the strain of the bacteria on melons and on equipment in the Colorado farm’s packing house. But investigators have not said how they believe the contamination occurred.

The first illnesses in the outbreak began appearing in August.

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