I think the airlines are fireproof on this one ( but I am NOT a lawyer).
Neither am I , but it seems that in some places ( e.g. Germany , south Korea, etc..) the authorities are spending lots of resources tracing back where the initial infection came from for every patient . Like the current evangelic church gathering in FRA which got 107 people infected . so if you can trace back the origin on an infection inside an particular aircraft /flight , the door will be wide open for litigation..