Critically ill patients with flu may receive a drug called oseltamivir. They may also receive medical therapies to support their lung function (extracorporeal membrane oxygenation; ECMO) and kidney function (continuous venovenous hemodialysis; CVVHD). CVVHD and ECMO may remove some oseltamivir from the bloodstream. The purpose of this study is to determine how much oseltamivir gets removed by CVVHD or ECMO in critically ill patients.