Stealth is the term used to describe techniques used to make malware inconspicuous – that is, to conceal any changes made by the malware to the infected system. This includes, for example, rootkits.
A stealth virus can infect a computer system in a number of ways: For instance, when a user downloads a malicious email attachment; installs malware masquerading as programs from websites; or uses unverified software infected with malware. Similar to other viruses, it can take over a wide variety of system tasks and can affect the computer's performance. When performing such tasks, antivirus programs detect the malware, but the stealth virus is designed to actively remain hidden from antivirus programs. It accomplishes this by temporarily moving itself away from the infected file and copying itself to another drive and replacing itself with a clean file. The stealth virus can also avoid detection by concealing the size of the file it has infected.
You can detect the virus by starting the system via a disk boot — to avoid systems the virus has control over — and then beginning an antivirus scan. However, even if detected here, there is a chance the virus has copied itself into another file on the system, so it remains a challenging virus to fully eradicate. In general, the best countermeasure is to use strong antivirus software designed to detect viruses and their hidden counterparts.