Fresh air: Yes lots of germs circulate inside the cabin, but if the air is not changed to fresh air (m3 per person) then condensation may occur on the heavily glazed windows and more importantly the carbon dioxide rich, second hand air that humans expel must be replaced with oxygen rich air at a rate faster that it is consumed, otherwise we will full unconscious.
The Temperature of the air outside at 35,000 feet is a lot cooler then at sea level, so this needs to be warmed* for comfortable inhalation.
The Pressure of air at altitude is a lot thinner than at sea level, and this also needs to be increased in pressure to allow our lungs to work at normal capacity.
The answer to all of these 3 factors was solved many years ago and understandably involves lots of engineering to provide what is abundant at sea level and unavailable in the correct form at altitude. The following is an explanation from a passenger jet Captain.
"The air is tapped from the engine compressors and then ducted to the air conditioning units for cooling if required. As the air from the engines is very *hot it and so needs to be cooled via heat exchangers that use ambient air for cooling or air cycle machines that operate like turbo compressors. The tapped air drives the turbine, the compressor pumps the air into the cabin. At altitude where the outside air temp. can be 56 deg. C below, the hot air is not cooled as much. The pressurisation is achieved by controlling outflow valves to regulate the outgoing air. On the 747 they maintain 8.6 PSI differential which gives a 5000ft' cabin at 35,000ft. If just one cabin window blew out of a 747, cabin pressure would be lost." and a rapid descent would follow. This is a continuous process and a system with inputs (fresh air) and outputs (old air) and a process (humans breathing).
I found more information available at Boeing www.boeing.com/commercial/cabinair/ or at Lufthansa
www.lufthansa-technik.com/aircraft cabin air
