MIAMI (CBSMiami) – The Centers for Disease Control and Prevention said the strain of cholera in Haiti is beginning to evolve as some develop a little immunity from the bacterium, according to the Washington Post.

The evolution of the cholera strain was expected and typical of the disease, according to the CDC. The disease’s mutation could make it easier for Haitians to suffer through the disease a second time.

Since the initial outbreak of Haiti on October 20, 2010, there have been a total of 534,647 cases; 287,656 hospitalizations, and 7,091 deaths, according to the CDC.

The CDC said the two different strains of cholera don’t appear to differ in the severity or duration of illness they cause. The CDC labeled the first strain of cholera Ogawa and the second type Inaba, with those infected with Ogawa more vulnerable to both strains.

The CDC said two cholera vaccines from the World Health Organization protect against both the Ogawa and Inaba strains of cholera.

CDC researchers said the evolving disease requires continued public health surveillance by the National Public Health Laboratory and the CDC. Both are trying to establish a lab-enhanced surveillance system for a range of infectious disease and help direct prevention efforts in Haiti.