that is an excellent question, that lots of scientists ask me when they hear what I do!
The virus that we use is not exactly the virus that you can find in nature and that can infect normal cells. We modify it so that it kills only the cancer cells but not the normal ones. My research is exactly to make sure that this virus doesn’t harm normal cells.
A simple way to explain how the virus can kill only cancer cells, is to imagine a normal cell as a house with a locked door and a cancer cell as a house with a broken door that is always open. A normal virus is able to unlock the door of the house and enter, but if we modify the virus so that it can’t unlock the door anymore, then the virus we’ll only be able to enter in those houses where the door is already open. Once the virus is in a house with an unlocked door (a cancer cell), it can replicate, kill the cell and then go to infect another cancer cell.