Trojans are the way to spread the virus. Trojans disguise themselves as operating programs, often with the preservation of the original functionality. And once they’re in your system, they start performing destructive acts before they’re even detected. Some work only on hacker’s instructions, while others start charging the device with malicious software immediately.
Since malware is the source of the virus itself, Trojans can also be considered viruses. Given that the purpose of this malware is similar to the purpose of spreading the virus, it is safe to say that it is one of the types of viruses.