Is there a difference between 患者 and 病人? Both 患者 and 病人 can mean patient. When would you prefer one over the other?
患 = contracted (of disease); 者= the one who
  
  患者 = the one who contracted (disease) = patient



The term 患者 describes 'a patient of a specific disease'. For example: '癌症患者' (cancer patient), '皮膚病患者'(skin disease patient). 
We cannot use '患者' without referencing the disease



  病人 = sick person/ patient



The term 病人 is a general term for 'sick people/ patient'. You can use this term to designate a unwell person without mentioning the disease. For example "病人需要更多的休息" (sick people/ patients need more rest). *-doesn't matter what kind of sickness
It can be used interchangeably with 患者 when referring a patient of a particular disease. For example 癌症病人 (cancer patient), 皮膚病病人(skin disease patient)