She has an extremely rare but deadly disease that can only be cured if she contracts a virus from you for which you are the carrier. Because this virus is benign and causes no adverse symptoms, it is difficult for you to transmit the virus, though the rate of transmission is very nearly 100% if a person spends the night with a carrier in same room. However, the rate of transmission decreases precipitously if a carrier spends the night with more than one person in a room.

Thus, it is imperative for her health that she spend the night with you alone in a room.