How do I complete the Infected Farm?

If I am correct you should have a anti-serum in your inventory. (retrieved from lab). If you have done that I think the area is finished. more info here. If you want more details about if they are timed. there is a small steam discussion which adds some more info.

all choices

Reservoir

Yes - use 1 dose Ag Center Serum - log book
No 

I wont post anything from the discussion as it is somewhat spoiler heavy