Maybe Jenner told Rick that everyone is a carrier but Rick didn't really understand what that meant?
Also even though it has almost nothing to do with the group I'd like to find out what happened to the ISS Astronauts. If they weren't evacuated when the plague first started they could still be alive at this point since, at least by "TS-19" the plague only has been global for 63 days. They haven't even spent a full month at the farm yet.
Maybe the Astronauts would be in contact with survivor colonies, maybe some major islands survived totally intact like Hawaii or Guam.