A dropper is a program that has been designed or modified to "install" a virus onto the target system. The virus code is usually contained in a dropper in such a way that it won't be detected by virus scanners that normally detect that virus. While quite uncommon, a few droppers have been discovered. A dropper is effectively a Trojan Horse whose payload is installing a virus infection. A dropper which installs a virus only in memory is sometimes called an "injector".