What Are the Symptoms of Lassa Fever? The symptoms usually show up 1 to 3 weeks after you’re infected. Most people have mild ones like: Slight fever Feeling tired and weak Headache In about 20% of cases, the disease brings on more serious symptoms, like: Bleeding from places like your gums, eyes, or nose Trouble breathing Throwing up Swelling in your face Pain in your chest, back, and belly Shock