This is a trojan detection. Unlike viruses, trojans do not self-replicate. They are spread manually, often under the premise that they are beneficial or wanted. The most common installation methods involve system or security exploitation, and unsuspecting users manually executing unknown programs. Distribution channels include email, malicious or hacked web pages, Internet Relay Chat (IRC), peer-to-peer networks, etc.
Indication of Infection
Many of these Downloaders install other malware including viruses as well as other Trojans.
Additionally many of them are used to remotely install Adware packages onto the affected host machine for the purposes of gaining referral revenue from the Adware software vendor.
Please note: If Adware is installed via a Downloader it may install it "cleanly" with the relevant uninstaller included for the user to terminate this Adware, although frequently this is not the case.
Methods of Infection
N/A. Downloaders are not viruses, and as such do not themselves contain any method to replicate. However they may themselves be downloaded by other viruses and/or Trojans to be installed on the user's system.
Many of these additionally are mass spammed by the author to entice people into double-clicking on them.
Alternatively they may be installed by visiting a malicious web page (either by clicking on a link, or by the website hosting a scripted exploit which installs the Downloader onto the user's system with no user interaction.