As many as 16 people in the United States have now died after eating cantaloupe contaminated with listeria, making this the deadliest foodborne outbreak in the United States in more than a decade.

The U.S. Centers for Disease Control and Prevention said Tuesday that 72 illnesses, including 13 deaths, are linked to the tainted fruit. Three more deaths that may be connected are still being investigated.

The outbreak has been ongoing since July, and has resulted in a massive recalls of cantaloupes. But experts with the U.S. CDC say while the melons are out of stores, the death toll could continue to rise, because listeriosis can sometimes take up to 70 days to develop after a person has eaten a contaminated item.

Here’s a little bit of background on listeria:

What is listeria?

Listeria is a common bacteria found in soil, water, decaying plant matter and manure. A certain strain, called Listeria monocytogenes, can cause severe food poisoning. It was first discovered in the early 1980s and has been linked to a number of outbreaks since.

Most listeria outbreaks involve meat products, like the massive Maple Leaf Foods outbreak in 2008 that was traced to listeria in deli meat. Only a few outbreaks have been linked to fresh fruits and vegetables. This is the first listeria outbreak linked to whole cantaloupes.

What caused this latest outbreak?

For now, experts have no idea. The U.S. Food and Drug Administration has traced the outbreak so far to cantaloupes grown at Jensen Farms’ in Granada, Colorado. The strain involved in this outbreak has been found on equipment in the farm’s packing house. But it’s unclear how it got there.

The affected cantaloupes have all been recalled and it's not believed that any were ever distributed in Canada.

How many have been affected?

It’s difficult to say. Symptoms of listeriosis –- the disease caused by listeria infection -- can appear anywhere between two to 30 days after contact with the bacterium. Symptoms can sometimes take up to 70 days to appear. That makes tracing the infection back to eating contaminated cantaloupe difficult.

Becasue listeriosis has such a long incubation period, Thomas Frieden, director of the CDC, and FDA Commissioner Margaret Hamburg expect more illnesses could show up in the next few weeks. "We will see more cases likely through October," Hamburg said Wednesday.

Most of those who have died do far have been over the age 60, a group that’s typically at risk of complications from listeriosis.

Despite the high number of deaths, this strain may not be more lethal than other listeria strains. The death toll might be a reflection of the fact that the majority of those eating the infected cantaloupe are older Americans, William Marler, a Seattle lawyer who represents victims of food-borne illness.

“Sometimes in outbreaks, it’s the population that’s consuming the food that drives the numbers... In this instance, you’ve got a lot of people 60 and older who are consuming cantaloupe," he told The New York Times.

The infection can also lead to miscarriages, but tracing these miscarriages can be difficult for infectious diseases investigators, since many miscarriages are unexplained.

Does everyone who’s eaten contaminated food get sick?

The risk of developing listeriosis if you have eaten contaminated foods is small in most people, says the U.S. CDC. But for those who do develop foodborne listeriosis, it’s fatal in 20 to 30 per cent of cases. In fact, listeria is more likely to cause death than other bacteria that cause food poisoning.

The elderly, the very young and those with poor immune systems are at high risk. In some of these patients, the bacteria can move out of the gastrointestinal tract and into the spinal cord, leading to meningitis, which is often fatal. 

Pregnant women are also at high risk. In fact, they are about 20 times more likely to get listeriosis than other healthy adults. A pregnant woman who contracts listeriosis could miscarry within the first three months of pregnancy. If the disease occurs later in the pregnancy, the baby could be stillborn or acutely ill.

What are the symptoms of listeriosis?

The disease can cause flu-like symptoms, such as persistent fever, headache, nausea, vomiting, cramps, diarrhea, and constipation.

Can listeriosis be treated?

Antibiotics are often effective in killing listeria bacteria and treating simple infections. However, some infections result in complications that often can’t be treated.

What can I do to protect myself?

There is no vaccine to prevent listeriosis. The Canadian Food Inspection Agency suggests following proper food hygiene is the best prevention method:

• Wash raw vegetables and fruit thoroughly. Wash the rinds of melons before cutting
• Thoroughly cook or boil items vulnerable to listeria, such as hot dogs, poultry and eggs
• Avoid raw, unpasteurized milk or foods made from raw milk cheese
• Wash hands before, during and after handling food, especially raw meat
• Never keep foods at temperatures between 4 degrees Celsius and 60 degrees Celsius for more than two hours
• If pregnant, avoid soft cheeses such as brie and feta
• Since listeria can survive on foods stored in the refrigerator, be sure to sanitize the refrigerator regularly