While riding the water ski in the Canal levels, you come to a point where you fight a Combine Gunship before you can progress. After you destroy the gunship, look around that area and you will find two sewer tunnels, one of them has a grate which is ajar and you can slip through.

Sprint through the radioactive waste and exit through the hole in the tunnel to find a vorginaut with a sore throat. Talk to him many times to hear all he has to say.