Visiting shady websites such as a site that pretends to stream a popular movie but only if you download a certain video codec which is really a Trojan.Downloading infected applications or unknown free programs.Here are some common ways Trojan software spreads on your device: However, they can’t infect your device on their own-it takes user interaction to spread them such as opening a malicious email attachment or unintentionally downloading and running a fake file. Trojans can infect your computer in any number of ways: they can look like free software and music to browser ads to seemingly legitimate apps.