What Causes Herpes Simplex Encephalitis?

my sister has been told that she has herpes simplex encephalitis of the left temporal lobe.She is undergoing radiotherapy for cancer and this was only raised after she had a fall on the ice and banged her head.Consequently she now has no sense of taste or smell.The HSE was only picked up after a scan on her head after the fall.Are the taste & smell related to her injury or to the HSE?How has she caught this virus is it because her immune system is compromised due to cancer