Not only is flying unhealthy because you are breathing everybodies germs.
Now studies show it is the lack of oxygen that causes problems. Lack of oxygen can make you feel ill and may lead to DVT (deep vein thrombosis) a condition that they warn you against in the inflight magazine these days.
DVT is dangerous and the solution is to put you on blood thinners (rat poison) .
Telegraph | News | Low oxygen 'increases DVT risk for most air passengers'