The realization that colds can kill has renewed interest in finding vaccines and treatments. The trouble is that the common cold is caused not by one virus but by hundreds of different ones. This means a vaccine or drug that works against one of these viruses, or one family of viruses, is usually ineffective against all the others. What"s more, because colds are usually so mild, if treatmentscause even minor side effects they can be worse than the disease. Such treatments will never get approval for general use, which is why most companies instead focus on drugs that relieve symptoms. Nevertheless, some drugs and vaccines are being developed against the cold viruses most likely to turn nasty.A vaccine against Respiratory Syncytial Virus (RSV), a virus which can cause serious illness in young children and the elderly, is going through clinical trials. It consists of a weakened strain of the virus given as a nasal spray. A treatment for RSV infections, based on RNA interference, is also in development. However, treatments for specific viruses are useless unless your cold is caused by the virus in question—and doctors have no quick way to work out which virus is to blame for a cold. Systems to do this are under development, mostly based on looking for specific DNA or RNA sequences, but none are near to reaching the market. An alternative approach would be to keep taking drugs that prevent infection throughout the cold season, such as a derivative of the anti-smallpox drug cidofovir which has been shown to combat adenoviruses, viruses that can cause upper respiratory infections. But again, as adenoviruses are only responsible for a few percent of colds, the benefits hardly justify the expense and risk of side effects from remaining on a drug permanently. Short of everyone on the planet isolating themselves for two or three weeks, so existing cold viruses run out of hosts and die out, it is hard to see how we can ever defeat the common cold. Even then, new cold viruses would evolve in time from animal viruses. Some even question whether it is desirable to try to eliminate colds. "It"s blind speculation," says Joel Weinstock of Tufts University in Boston in the US, "but the common cold may protect us from more serious viruses." An occasional sniffle might be a price worth paying if it keeps our immune defenses primed.
单选题
The drugs against colds are usually ineffective because _____.