It sounds like some (not all by far) here may be flying on empty. It helps to be hydrated properly (and I suspect oral rehydration mix / salts is better than salt tablets), have some protein in the system).

Mild symptoms of motion sickness can often be dealt with by ingesting various forms of ginger - for some even Ginger Ale helps, for others, hard ginger candy or ginger in other forms can help, and it has significantly fewer side effects for some than medications.

The myth of scads of germs attacking our immune systems in the nasty aircraft cabin recyled air is pretty much rubbish (only as much as 50% of the air is "recycled", and it is filtered), though you certainly could catch something from a seatmate or contaminated surfaces. Among other sites, this JAMA article PDF link is instructive.