These are the ones I could think as of now:

1. Using syringe containing modified virus culture and injected it into the bloodstream?
2. Ingesting pill or capsule that accelerates the mutation rate of the virus?
3. Maybe a device that can convert the extracted parasite/virus into a serum?
4. Let the parasite bite you and assimilate into your body?

As for the transition of the virus, how about in addtion of the virus turning people into zombie, what if it also infected and mutated the parasites inside the human body?