Oseltamivir is a medication that decreases the spread of influenza A and B viruses, the viruses responsible for the "flu." Neuraminidase is an enzyme that enables influenza virus to spread from infected cells to healthy cells. Oseltamivir blocks the action of neuraminidase (it is a neuraminidase inhibitor) thereby reducing the spread of influenza. By preventing the spread of virus from cell to cell, the symptoms and duration of influenza infection are reduced. On average, oseltamivir reduces the duration of symptoms by one and a half days if treatment is started within forty-eight hours after symptoms begin.