听力原文： Many people catch a cold in the springtime or fall. It makes us wonder if scientists can send a man to the moon, why can't they find a cure for the common cold? The answer is easy. There're actually hundreds of kinds of cold viruses out there. You never know which one you will get, so there isn't a cure for each one. When a virus attacks your body, your body works hard to get rid of it. Blood rushes to your nose and causes a blockage in it. You feel terrible because you can't breathe well. But your body is actually eating the virus. Your temperature rises and you get a fever. But the heat of your body is killing the virus. You also have a running nose to stop the virus from getting to your cells. You may feel miserable but actually your wonderful body is doing everything it can to kill the cold. Different people have different remedies for colds. In the United States and some other countries, for example, people might eat chicken soup to feel better. Some people take hot baths and drink warm liquids. Other people take medicines to relieve various symptoms of colds. There's one interesting thing to note. Some scientists say taking medicines when you have a cold is actually bad for you. The virus stays in you longer because your body doesn't develop a way to fight it and kill it. According to the passage, why haven't the scientists find a cure for the common cold?