POTS is caused by a malfunction of the autonomic nerve system, which governs many of the body's involuntary actions. Because many ME/CFS patients have difficulty standing and have symptoms such as dizziness, impaired vision, nausea, and exhaustion, it has been proposed that some of them may also have POTS. The autonomic nervous system controls various organs of the body without direct control from the brain. It consists of two branches: the sympathetic branch and the parasympathetic branch. The sympathetic branch is responsible for "fight or flight" responses such as increased heart rate, dilated pupils, and sweating. The parasympathetic branch helps to restore balance after stress has been released and functions such as digestion and reproduction are restored. When the sympathetic branch is activated too much, it can lead to several problems including rapid heart rate, high blood pressure, and diarrhea. This is called "sympathoadrenal activation". Patients with severe cases of this problem may need medication to reduce their body temperature or they may even require a pacemaker because their heart muscles cannot handle a normal heartbeat. In patients with CFS/ME, the sympathoadrenal axis is often overactive, which can lead to many other problems beyond just POTS. For example, it has been suggested that excessive adrenaline in these patients may cause fever-resistant bacteria to grow in their bodies, which could lead to multiple serious infections.