As others have said, use a decongestant, ie psuedoepherine (Sudafed) 60mg every 4-6 hrs. Benadryl is fine for a sleep aid as well. Keep hydrated, no carbinated or alcoholic beverages.

As far as other "sick" pax on-board, the airflow in airliners is not from front to back or back to front. The air you breathe is more or less exchanged with the 2 rows in front of and behind you, and most airlines use HEPA filters. Of course if your seatmate sneezes all over you, the afore mentioned things aren't of much consequence. If you want more info on cabin air quality, you can visit my website (International Health and Travel Medicine) at http://ihtm.11net.com/tw.htm