[If you ever feel sick on the plane...]
Economy class syndrome was coined in the late 1990s when it turned out that people who has traveled long distances by aeroplane were at an increased risk for thrombosis, especially deep venous thrombosis and its main complication, pulmonary embolism. Although all these diseases had been recognised for a long time, the possibility of litigation against airline companies brought them into the limelight when this "syndrome" was reported.
The mechanism for thrombosis in travellers is probably due to a combination of immobilisation, dehydration and underlying factors. Patients with disease that predisposes them for thrombosis, such as antiphospholipid syndrome or cancer, are probably at a much greater risk.
Prevention consists of adequate hydration (drinking, abstaining from alcoholic beverages and caffeine), moving around and calf muscle exercises. In patients with a known predisposition for thrombosis, aspirin is often prescribed, as this acts as a mild anticoagulant. Severe risk for thrombosis can prompt a physician to prescribe injections with low molecular weight heparin (LMWH), a form of prophylaxis already in common use in hospital patients.