HIV greatly affects the immune system. When the immune system is under attack it can cause a person to feel quite tired and even lethargic. Feeling fatigued can be a sign that comes quite early when exposed to HIV and is also a later sign of the disease. Feeling fatigued during the early stages of the infection may not be as noticeable.
It is important to note that fatigue is different from just being tired. When a person is feeling fatigued they often feel like they cannot even move without becoming quite exhausted. Fatigue can be a symptom of many illnesses including the flu or mono, which is why it is often difficult for a person to know exactly what is going on. It is often best to see a doctor to be tested for HIV and other illnesses if you are experiencing chronic fatigue.