If my main and only sim in the household, who im going through the story with, is infected; can I still play through the whole story without needing the help of a another sim to fix her and/or also so...
If your sim is infected, they won't be posessed all the time. The posessed moodlet lasts for 3 hours and after that your sim is normal. The infected trait makes sims posessed random times, but it won't be often. It happens about twice a day. It does offer benefits though. If your sim is posessed, all their needs will be filled to the max and won't decay while they are posessed.
The disadvantage is that infected sims cannot fight the mother plant. The good news is that once you discover the mother plant, you can start making vaccines, which will cure your sim and allow them to fight with the mother plant. Interesting thing to note: If your sim dies after losing a fight with the mother plant, their ghost's needs will never decay.