A (1) is a program, made by either a (2) or not. It is not so easy to make one, but (3) people know how to do it, for example, you may know that you have to know some exact (4) languages to make a computer virus . A virus’ (5) is to make a computer be ill, even (6) down, as the normal viruses. If a virus get into your computer, you have to (7) it immediately before getting in (8) because viruses are very dangerous! And the (9) is that there are a lot of kinds of viruses (normal viruses, Trojans etc.). If you notice that your (10) “sticks” (do not make the movements you want as you want) many times or your computer shows you messages like “This program has made an illegal operation and ... ” or so mething like this, begin to wonder and call an expert to help you!