Colorado's famous Rocky Ford cantaloupes are being blamed for an outbreak of Listeria that has killed four people and sickened dozens of others in six states. No formal recall has been issued, but health officials are advising people at high risk of contracting the bacterial infection—the elderly, pregnant women and those with compromised immune systems—to steer clear of the Colorado melons, the Denver Post reports. Officials are scrambling to locate the source of the outbreak, and some Colorado chains have pulled their supplies as a precaution. (More listeria stories.)