a general feeling of weakness, ill health, or uneasiness/discomfort that doesn't have an identifiable cause and is often a sign of approaching illness;
e.g. He was experiencing exhaustion and a sense of malaise for a few days before he actually felt sick.
(noun)
a nonspecific sense of ill-being, often in a moral and/or mental sense;
e.g. They were all filled with malaise from being under so much strain.