If you’re a healthy person, there’s a chance you’ve been infected with a bacterium called Listeria [Lis-TEAR-ia] sometime in your life without realizing it. But if you’re pregnant or otherwise immunocompromised, you’ve probably been advised to steer clear of soft cheeses, uncooked hot dogs and deli meats because of concerns that the bacterium could induce a strong immune response, leading to miscarriage, pre-term labor or even death.

At least two-thousand people become seriously ill each year… and five-hundred more die… from eating foods contaminated with Listeria. But even dangerous pathogens sometimes have a soft side: Listeria may be the next big tool for treating immunocompromised patients with cancer and H-I-V.

Scientists have engineered less-dangerous versions of the bacteria that they hope will stimulate and strengthen the immune system without causing life-threatening disease. In patients with cancer, the bacteria can enter cancer cells and send signals to the immune system to selectively destroy tumors.

Scientists are also engineering Listeria capable of inducing specific immune responses against virus particles in patients with H-I-V and hepatitis. This boost may be exactly what immunocompromised patients need to fend off their infections.

Scientists warn that there is a potential downside to this approach: by disrupting the delicate balance between inducing a helpful immune response and causing disease, their efforts have the potential to backfire. But while it may be several years before the vaccines are available, safety trials have so far yielded promising results.