My Significant Other Has Had A Cough Now For About

My significant other has had a cough now for about 2 months maybe a little longer. The cough is a dry cough. he also has hot and cold flashes off and on and sometimes night sweats that soak the sheets completely. The night sweats started before the cough. The night sweats are not consistent and come and go as do the hot and cold flashes. He fatigues easy. Complains of his calves and feet being sore. When the cough started he had fever, achiness, congestion and diarrhea. Almost all the symptoms have gone away without seeking medical treatment except the fatigue, night sweats, cough, sore calves and feet. What can this possibly be?