Computer viruses and worms pose serious threats to the modern society by stealing the confidential information, modifying files and destroying the operating system. Especially, the email has become the primary measure to spreading viruses. Inspired on the classical epidemiological model, an improving model based on the dynamic infected rate and immune rate is proposed. The simulation experiments are carried out by altering the correlative parameters and the experimental results provide the clue for developing the detected scheme of email virus. A promising detected scheme is proposed which detected viruses based on the abnormal behaviors of viral emails rather than the pattern of viruses. This scheme can throttle the new viruses and accords with the developing requirement of anti-virus technology.