Instead of a single causal agent, some scientists now suspect that several conditions may need to be met before the full-blown syndrome can occur. One hypothesis holds that people who become ill may have encountered one or more causal agents at a time when they were especially susceptible to an attack.
Researchers are currently following several promising avenues. For example, they are trying to determine whether CFS patients respond differently than healthy people do to strenuous exercise testing and whether they have more dramatic allergic reactions. They are also comparing the activities of certain hormones and of cytokines (chemical messengers produced by the immune system) in CFS patients and healthy people.
There is no convincing evidence that the illness is contagious in the true sense of the word, even though it may be triggered by an infectious agent. Although only a small proportion of patients regain the level of health they enjoyed before developing CFS, very few get progressively worse.
In recent years, many studies have found a wide variety of measurable abnormalities involving infectious agents and the immune systems and brains of people with CFS. While none of these abnormalities have been found in all patients with the illness, they indicate that CFS does indeed have real physiologic features. In addition, most of these abnormalities are not seen in mental illness, making it unlikely that CFS is a psychiatric disorder.
The ailment has sometimes become a catch-all label for symptoms that cannot be otherwise explained. In reports and interviews, more than a dozen experts cited a trend in which many doctors have come to believe the syndrome is real, although most remain skeptical or are yet to be persuaded. Among those who are convinced that CFS is a real disease, three principal theories are being pursued.
One theory is that any of a number of infections agents and possibly chemicals can provoke the immune system to counterattack and somehow keep it in a lasting state of activation. According to this theory, the victims of chronic fatigue are those who cannot get rid of common infectious agents the way most people do, perhaps because of genetic differences. The result is a permanently activated state in which the immune system stays in high drive, as if to combat a continuing viral infection.