In the book I'm writing a there is group of researchers who are trying to cure a pandemic disease.

For plot reasons the cure has to be a retrovirus. My problem is how to administer this retrovirus.

My first thought was injection, but due to the setting obtaining a reliable supply of clean needles would be nearly impossible.

So, now I'm thinking some kind of concentrated drops administered orally/sublingually. Could this be a plausible means of transmitting a retrovirus? If not would eye drops or nasal drops be any better?

Depends on the specific retrovirus as different ones have different preferred methods of infections. I'd say eye drops would probably be more effective than the oral route (unless there are oral lesions present).

 

For example, the retrovirus HIV is transmitted more easily when there's access to the blood stream. Chance of infection orally is extremely low unless there are at least small cuts in the mouth.