Symptoms of acute mountain sickness (AMS) and infection were recorded daily in 283 hikers walking the Mount Everest base camp trek in the Nepal Himalaya. Some 57% of subjects developed AMS, and 87% experienced at least one symptom of infection during the study period. Coryza (75%), cough (42%), sore throat (39%), and diarrhea (36%) were especially prevalent. All symptoms of infection were more prevalent among those with AMS. The incidence of AMS was greater among those with more symptoms of infection (p = 0.00004), and the number of symptoms of infection experienced with positively correlated with AMS score (rs = 0.43, 95% CI = 0.33 to 0.52). These results suggest that symptoms of infection are common at high altitude and are associated with a higher incidence of AMS. People with infections should ascend at a slower rate at high altitude.