As fall gives way to winter, we all know what that means ... tis the season! And we're not talking holidays -- we're talking flu season. Ever wonder what's going on inside your body when it's under attack? Just how does your body fight back to stop a virus from advancing. Every day, trillions of cells inside your body engage in a life and death struggle with viruses and a variety of enemy intruders. This episode of Curiosity presents that battle in all its surprisingly stunning glory as it explores the once invisible world of the human cell. Based on the latest scientific research -- combined with new imaging and CGI technologies -- we expose this world like never before by revealing a human cell's 48-hour battle to the death with a virus.