It's not directly a trojan or virus, but works by getting into the system and then inviting all sorts of other nasties to join in. They could be virus, trojan or botnet types.
Eventually so many are allowed in that your system cannot function = rebuild required.
They can be removed but the last one I sorted out for somebody, involved 6 scans at 3-4 hours a pop, and I had to use 3 different types of of AV software, plus some specific removal s/ware.
Hard work.