将句子连成文章Put the following group of sentences into correct ord

将句子连成文章
Put the following group of sentences into correct order to make a coherent paragraph.Write down the number of the rearranged sentences.
1.Scientists cannot identify one virus as more responsible for causing colds than other viruses.
2.Scientists know that it is caused by a virus,not bacteria.
3.If you are vaccinated against virus A,you are protected from virus A,but not from viruses X,Y,and Z.
4.Therefore,it is difficult for them to develop a vaccine to prevent colds.
5.In fact,there are probably over one hundred different types of cold viruses.
6.They also know that the cold virus is very small (an electron microscope is needed to see it),very durable,and very difficult to identify.