Wi-Fi syndrome does, however, make sense in the context of a larger phenomenon: the “nocebo effect,” the placebo effect’s malevolent Mr. Hyde. With placebos (“I will please” in Latin), the mere expectation that treatment will help brings a diminution of symptoms, even if the patient is given a sugar pill. With nocebos (“I will harm”), dark expectations breed dark realities. In clinical drug trials, people often report the side effects they were warned about, even if they are taking a placebo. In research on fibromyalgia treatments, eleven per cent of the people taking the equivalent of sugar pills experienced such debilitating side effects that they dropped out.

The nocebo effect is not confined to clinical trials. After the 1995 Aum Shinrikyo sarin nerve-gas attack in Tokyo, for example, hospitals were flooded with patients suffering from the highly publicized potential symptoms, like nausea and dizziness, but who had not, it turned out, been exposed to the sarin. This is common in disasters where the agent is invisible, as with chemicals or radiation. At the extreme are the occasional outbreaks of mass symptoms with no discernable physical cause, such as a famous case at a Tennessee high school that was evacuated after a teacher reported a “gasoline-like” smell and feelings of dizziness. About a hundred students and staff were taken to the emergency room, and thirty-eight were kept overnight. An extensive investigation found no evidence of any chemical presence, and researchers have since concluded it was a “mass psychogenic illness.”

As for Wi-Fi syndrome, a recent analysis of forty-six studies involving nearly twelve hundred volunteers concluded, similarly, that the signals do not cause the symptoms. In one experiment, researchers in Austria had people spend several nights sleeping in a cocoon of custom-engineered bed netting, but found it made no difference whether or not the netting stopped electromagnetic signals. One careful study of mobile-phone waves found a significant pattern of headaches, but it turned out the headache cluster fell in the control group—those who had not been exposed to signals. (There is also no convincing evidence of a link between cell phones and brain cancer, another common fear.)

I spoke recently with Michael Witthöft, a German scientist who has been looking into medically unexplained syndromes for more than a decade. The results of his latest inquiry, conducted with a colleague at Kings College London, should make any journalist cringe. Witthöft recruited volunteers, gave them a bevy of psychological tests, and then divided them into two groups. He showed one group a BBC broadcast about the “dangers” of Wi-Fi, featuring all the usual tropes of scare TV: ominous music, uncritical interviews, alarmist narration, and jarring cutaways to cell-phone towers. Witthöft showed the second group a program on mobile-phone security. Then each volunteer was brought into a small room, seated in front of a computer, and fitted with an ungainly headband holding a silver antenna described as a “Wi-Fi amplifier.” They were told to push a button—a red Wi-Fi symbol flashed on the screen—and wait fifteen minutes.

There was, in fact, no Wi-Fi in operation, but Witthöft still observed dramatic effects. Sitting in the room with the (fake) Wi-Fi caused tingling in fingers, hands, and feet; pressure and tingling in the head; stomachaches; and trouble concentrating. Two of the subjects found the experience so unpleasant that they had to stop before their time was up. (Only the more anxious volunteers who saw the scare TV reacted badly.) Witthöft’s work, and a similar experiment just published on wind-turbine complaints, draws a direct line between irresponsible journalism and health problems. Following a first scare, Witthöft told me, some people seem to get caught in cycles of negative reinforcement. They experience physical symptoms, leading them to pay closer attention to how their body feels. Hypervigilance leads them to notice more symptoms—is that a new tingle?—and become more alarmed. They then withdraw in an effort to avoid what ails them, which can lead to depression, which can itself aggravate the symptoms. In the worst case, they might head for the nearest cave.

“Placebo” has come to mean fake, but, as Michael Specter explained in a feature last year, that’s not quite right. The effects are real—so real, in fact that some scientists argue that doctors should receive more training in using placebos, and make them a regular part of their practice. Patients given placebos experience biochemical changesthat improve their condition. Placebo painkillers activate the body’s natural analgesics. Parkinson’s placebos prompt the brain to release dopamine; anxiety and depression placebos elicit changes in the areas of the brain that regulate emotion. One particularly remarkable study recruited patients with irritable-bowel syndrome and told them that their treatment would be “pills made of an inert substance, like sugar pills, that have been shown in clinical studies to produce significant improvement in I.B.S. symptoms through mind-body self-healing processes.” Even though the treatment was a placebo, and even though the patients knew it was a placebo, they showed significant improvement.

With the recognition of the nocebo effect, though, some doctors now speak of an odd ethical dilemma: the Hippocratic Oath dictates “do no harm,” but being honest with patients about potential side effects increases the odds that they will experience them. One suggestion is to adopt an ignorance protocol: ask patients for their permission not to tell them about minor drug side effects. A less fraught alternative is for doctors to speak more carefully, working hard to put the negatives into their proper place. If doctors say “the great majority of patients tolerate this treatment very well” before giving a flu shot, patients experience fewer “adverse events.” The larger problem lies outside the clinic: the Internet has become a powerful—and, to some, irresistible—nocebo dosing machine. In another day, it took weeks or even months for a person to gather enough reading to become very, very afraid. Now one can achieve a state of dread in a few short hours, surrounded by the comforts of home.