Is my fatigue caused by depression or something infectious?

Only exam with tell. This requires a complete evaluation by a physician. There are chacteristics of both sharing only fatigue as a single symptom. Extensive testing is only indicated after a good history and physical.
Hard to tell. Fatigue is a broad symptom across many ailments. I would recommend contacting your primary care physician for a complete work up. If there's no medical reason for fatigue, you might want to re-evaluate your daily routine -- exercise, nutrition, self-care, fun activities to help you understand yourself better. You might want enlist help of a psychotherapist to guide you through the process.