This is because they open and close files to scan them and this gives the virus additional chances to propagate. These same scanners will also fail to detect stealth viruses, because the act of opening the file for the scan causes the virus to temporarily disinfect the file, making it appear normal. Trojan Horses are not viruses. Trojan Horses are generally difficult to detect. Symantec has issued a Trojan Horse detection engine as an update to its Norton AntiVirus 4.0 and subsequently released products.
Computer Worms are programs that spread themselves from computer to computer over a network.
Worms, unlike viruses, do not infect programs, diskettes, or files with macro capabilities. Instead, they make copies of themselves and send these copies over the network to other targeted machines. Like viruses, Worms come from anonymous or untraceable sources. Worms are often equipped with dictionary-based password crackers and other cracker tools that enable them to penetrate more systems. Worms often steal or vandalize computer data.