Health officials are reporting the first U.S. case of a human infected with a "superbug" – an infection that leaves its victims with few, if any, options for treatment. According to the Centers for Disease Control and Prevention, a 49-year-old Pennsylvania woman was diagnosed with a rare strain of E. coli that proved to be something called plasmid-mediated colistin resistance, or mcr-1. Here's a quick look at the "superbug" and what mcr-1 is. What is mcr-1? Mcr-1 is a colistin-resistance gene. What that means is that the strain of E. coli the woman contracted carries a gene on a portion of DNA called a plasmid. The plasmid passes along that gene -- mcr-1 -- making more of the bacterium in her body resistance to antibiotics. This is especially concerning since colistin is considered a drug of last resort -- one used when other antibiotics fail. How did she get mcr-1? That is not known right now. The woman has not traveled outside of the United States during the past five months,