Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) The US Centers for Disease Control and Prevention confirmed Friday that pet rats are the source of an outbreak of Seoul virus infections in Illinois and Wisconsin. The virus has been confirmed in eight patients in an ongoing investigation. The recent cases are "the first human cases we've seen in the United States associated with pet rats," said Dr. Jennifer McQuiston, a veterinarian and deputy division director for CDC's division of high consequent pathogens and pathology. Several previous outbreaks reported in the US occurred in wild rats. "There was an outbreak reported in Europe previously associated with pet rats, so it's not the first time this has been associated with pets worldwide," McQuiston said.The CDC confirms the US' first human cases of Seoul virus associated with pet ratsSeoul virus is similar to hantavirus 
 Question:"It's been fairly complicated," said McQuiston, adding that the _ believes so far that the virus has not spread beyond or outside "the network that we're investigating right now."

CDC