Biological virus can attach itself to a human host cell and take charge, using the cell's functions to make the substances needed to form new virus particles, which then leave that cell and spread, repeating the process in other cells. (2) Biological viruses cause many diseases—some minor, like the common cold; but some life-threatening, like polio or AIDS. (3) Biological viruses may kill the host cell or make the cell itself malignant, or the virus may set off a dangerously violent response in the immune system. (4) Biological viruses reproduce and spread in various ways, and they may be very hard to treat because they can take forms that the immune system cannot detect. (5) Computer viruses are programs designed to attach themselves to ordinary software, take it over, and then reproduce and spread. (6) A computer virus can do its damage by attacking the startup program, at which point antivirus devices cannot yet detect it; or by attacking the operating system; or by attacking applications such as databases. (7) In any case, the virus can distort or kill computer memory. (8) A computer virus, as the name implies, is very much like a biological virus. The sentence that expresses the main idea is: (Choose the number of the sentence. Then click "GO.")