About 50 people have been reported infected in an E. coli outbreak involving McDonald's Quarter Pounder sandwiches.

The outbreak began around Sept. 27 and no infections have been reported in California as of yet. But in addition to Nebraska and Colorado, infections have been confirmed in Oregon, Montana, Wyoming, Utah, Kansas, Missouri, Iowa and Wisconsin.

So far, around 50 people have been reported infected, with 27 infections occurring in Nebraska and nine in Colorado, according to the CDC.

Officials estimate that the actual number of people infected is probably much higher as it usually takes three to four weeks to determine if a sick person is part of an outbreak, and also because many people recover without being tested for E. coli.

In response, the fast food chain has proactively removed the onions and patties used for the burgers from stores in the affected states, according to the CDC. As a result, Quarter Pounder hamburgers will be temporarily unavailable in some states.

Most instances of E. coli contamination are caused by poor sanitation practices, such as employees not washing their hands, crops being grown too close to farms where animals live and contaminate the soil and runoff with feces, and companies' failure to properly process and test foods, according to Teresa Murray, consumer watchdog director for the Public Interest Research Group.