В 3-м абзаце найдите предложение с инфинитивом в функции обстоятельства цели.
В 5-м абзаце найдите предложение с инфинитивом в функции определения.
3. Viruses spread when people distribute infected files by exchanging disks and CDs, sending e-mail attachments, exchanging music on file-sharing networks, and downloading software from the Web. To avoid viruses, be cautious of floppy disks, homemade CDs, and Web sites that contain games and other supposedly fun stuff. Check these files with antivirus software before you copy or use them.
5. A key characteristic of viruses is their ability to "lurk" in a computer for days or months, quietly replicating themselves. While this replication takes place, you might not even know that your computer has contracted a virus; therefore, it is easy to inadvertently spread infected files to other files to other people's computers.