If left untreated, could a viral infection become something more serious?

Please get a second opinion. This might also be pneumonia like you mentioned. You don't have to be old or have a weakened immune system either for a viral infection to cause a secondary bacterial infection. Take walking pneumonia for just one example. Healthy people do die from this because of opinions like this and that sometimes they don't even know that they are sick at all. The antibiotics will keep any bacteria away while the symptoms are treated otherwise. Then the right diagnosis will tell you exactly what and possibly how long you might be ill. Drinking plenty of water, and rest will in truth help your immune system fight this off, but if she didn't take blood work then how did she even know that it was viral is my question. Knowledge can only take you so far, that's why there are tests that can confirm the doctors findings. In my opinion, this is the only way a proper diagnosis can be made. Good luck and God Bless