  One of the most promising strategies to treat cancer is attacking it with
viruses. Virus can kill tumor cells specifically or act as carriers that
deliver normal genes into cancer cells. A model for virus therapy of cancer is
investigated and some of its predictions are in agreement with results obtained
from experimental tumors. Furthermore, the model reveals an oscillatory
(periodic or aperiodic) response of tumor cells and virus populations which may
difficult clinical prognosis. These results suggest the need for new \textit{in
vivo} and \textit{in vitro} experiments aiming to detect this oscillatory
response.
