Several people who were injured when a tornado devastated Joplin, Mo., last month have become sickened by an uncommon, deadly fungal infection and at least three have died, although public health officials said Friday that a link between the infection and the deaths was not certain.

Also on Friday, the death toll from the tornado was raised to 151.

Eight tornado victims have fallen ill from the mysterious infection, and each had “multiple injuries and secondary wound infections,” said Jacqueline Lapine, a spokeswoman for the Missouri Department of Health and Senior Services. Citing confidentiality rules, officials declined to discuss the treatment or condition of the patients.

The fungus that causes the infection, which is believed to be mucormycosis, is most commonly found in soil and wood, according to the Centers for Disease Control and Prevention, which is studying samples from the eight Joplin patients. “It is a very aggressive and severe infection,” said Dr. Benjamin Park, chief of the epidemiology team in the C.D.C.’s Mycotic Diseases Branch. “It is also very rare.”

Mucormycosis enters the body either via a puncture wound or when a victim breathes in its mold spores, officials said. Those who have weakened immune systems have a mortality rate as high as 90 percent. Other people at risk include those with diabetes or cancer and burn victims.