So a computer virus that is harmless to conventional computers because it specifically targets the 'quantum part', but specifically malicious.

Do you mean any malware or are you specifically asking about a virus, i.e. some form of self replicating malware?

I am ignorant of the status of quantum computing but I am assuming that there are generally not networks of quantum computers open to the internet so I am of the persuasion that the malware would be a very target attack against a specific computer which is likely not engaged in any commercial or retail tasks but rather in research or some state sponsored activity, in which case publication of such a targeted attack seems less likely.

I.e. this won't be someone opens up a suspicious email attachment on their quantum computer.

Unless this is more of an Messenger Theory attack, al a Adromeda Evolution, where existing networks are infected with selfreplicating malware that is harmless/dormant until it successfully finds itself on a quantum computer, waiting for the day that quantum computers are commercially available.

What about a virus targeting the non-quantum part of a system that includes a quantum computer? I find it highly unlikely the quantum part is targetable, but it is also very likely that any "quantum computer" hardware will include a substantial "traditional" computer that controls the quantum part..

@MartinModrak Must target the quantum part

What if a researcher programs one as a proof-of-concept and never releases it?

@soweliSon Has to be a real virus identified as having malicious intent