If you are pregnant or know anyone who is pregnant, you have almost certainly heard of Listeria, a dangerous bacterium that contaminates vegetables, dairy and meat. It is something you want to avoid: Listeria infections kill about 500 people a year in the U.S., and 2,000 more become seriously ill with food poisoning. Pregnant women are about 20 times more likely to become infected.

So it might surprise you that scientists want to inject patients who have HIV and cancer with the bacterium, whose scientific name is Listeria monocytogenes.

That is because Listeria creates a strong immune response in the body. Medical researchers are trying to harness this behavior by creating their own modified version of Listeria that is less dangerous as a bug, strengthens the immune system in general, and has the ability to get inside tumors. Once inside the tumor, the idea is for this bioengineered Listeria to give the immune system the green light to terminate the tumor with extreme prejudice.