It's important however to understand the distinction.
Viruses are defined by their self-replication code. They are infectious and simply inserting a USB stick can in rare cases infect it. On the other hand, even free software now has better than 95% detection rate for unknown
viruses and better than 99% overall. Viruses can be easy or difficult to remove, but their removal, if successful, is easily confirmed.
Trojans aren't infectious. There is no need for special precautions with an affected PC, even using separate boot media and such, and files can be freely transferred from the affected machine. A trojan can only be acquired from a dedicated source, not from an infected PC.
Since trojans have no giveaway code, their detection a toss, as is removal. It is often a question whether a particular program even is a trojan or not, and it can hinge solely on how reputable an entity operates it.
Most viruses have been written as personal exercises, to do damage, let off steam, spread infamy. They are a dying breed due to high detection rate, though. Trojans today are almost always written by hired guns and run for profit. Milder ones just want to show you ads, hardcore rootkits can log your banking details or hijack your PC to use it as part of Botnet.
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