Hi Sofja, Sep and Collin,
– Airplanes can spread influenza and other viruses, mainly because they can transport infected people over very long distances in a short amount of time. The massive increase of airtravel over the past 100 years have definitely increased the risk of larger outbreaks and pandemics. Infection during the flight, on an airplane itself is quite rare howeve. This is because the airflow is very strictly regulated on an aircraft and the air is blown downwards, causing any droplets coming from a person’s nose or mouse to quickly fall towards the ground, where it is difficult to reach another person’s mouth or nose and infect them. In addition, the air is passed through a HEPA filter, which very efficiently removes viruses and bacteria from the air.
– If by ‘average’ you mean young and healthy and by flu you mean the seasonal flu that is circulating right now, then no. Complications in these types of people do occur occasionally, but it is very rare. There are much more dangerous strains of flu however, such as birdflu (H5N1) that has a very high mortality rate in humans. Luckily, H5N1 is not very good as spreading from person to person (yet) and therefore typically only people working with poultry can get sick when there is an outbreak among the birds.