TOEFL Reading練習 #020 — The Fever That Fights Back(日本語解説)

The Fever That Fights Back

¶1 A fever feels like something has gone wrong, an alarm the body sets off by accident while fighting off an infection. For most of the twentieth century, medicine mostly treated it that way, reaching almost automatically for drugs that bring the temperature back down. Research since then complicates that picture. Raising body temperature appears to be something the immune system does on purpose, and forcing it back down can sometimes work against the very process it is meant to support.

¶2 The oldest evidence for this idea is also the strangest. In the early 1900s, patients with advanced, untreated syphilis often developed severe damage to the brain and nervous system, and no cure yet existed. The Austrian physician Julius Wagner-Jauregg noticed that some of these patients improved after a bout of an unrelated illness that produced a high fever. Starting in 1917, he began deliberately infecting patients with malaria, a disease that causes repeated waves of intense fever, and then curing the malaria with quinine once the syphilis symptoms eased. Many patients recovered enough to leave the hospital, and Wagner-Jauregg won a Nobel Prize for the treatment in 1927. What his method did not explain was why it worked. Because so many things change in a sick body at once, doctors could not tell whether the fever itself deserved the credit or whether some other effect of the malaria infection did. That question would take decades, and a very different kind of experiment, to settle.

¶3 The explanation that gradually emerged centers on a chemical signal called a pyrogen, a substance released by immune cells during infection that travels to the hypothalamus, the region of the brain that manages body temperature. Pyrogens do not simply let temperature drift upward; they reset the hypothalamus's target level the way a hand turning up a thermostat resets a room's target temperature, and the body then works to reach that new, higher setting by shivering and reducing blood flow to the skin. Many disease-causing microbes reproduce more slowly at higher temperatures, and several types of immune cells work faster, so the shift in the body's target temperature is not incidental to fighting infection. It is one of the tools the immune system uses to fight it.

¶4 Two very different kinds of evidence eventually confirmed that the temperature increase itself, not just the underlying infection, drives recovery. The first came from cold-blooded animals such as lizards and fish, which cannot generate a fever internally and must seek out warmth from their surroundings instead. [A] In the 1970s, the physiologist Matthew Kluger infected desert iguanas with bacteria and let them choose their environment; infected iguanas consistently moved to warmer spots and raised their body temperature by several degrees, a response now called behavioral fever. [B] When Kluger prevented infected iguanas from reaching the warm areas, far more of them died than those that were allowed to warm themselves. [C] The second line of evidence came from controlled trials in which infected animals were given drugs that block the fever response; animals whose fever was suppressed this way typically took longer to clear the infection or recovered at lower rates than animals left to run a fever on their own. [D] Because two unrelated methods, one relying on an animal's own behavior and the other on directly blocking the response, pointed to the same conclusion, researchers grew confident that fever is not a side effect but a cause of faster recovery.

¶5 This shift in understanding has changed how many doctors think about a mild fever. Rather than treating every rise in temperature as a problem to eliminate immediately, some now recommend watching and waiting through a low-grade fever, on the reasoning that the body may be doing useful work. That reasoning has a limit, however. The same heat that slows down invading microbes and speeds up immune cells can, past a certain point, damage the body's own proteins and cells. In humans, temperatures approaching 41 degrees Celsius move from beneficial to dangerous, so the value of a fever depends entirely on staying inside that range. Wagner-Jauregg's patients were pushed close to that edge under careful medical supervision; left unsupervised, the same fever that helped them could just as easily have caused the harm it was meant to prevent.