The rapid advancement of quantum computing in recent years has been marked by the construction of increasingly larger machines and their integration into the internet as cloud-based quantum computingCloud-based quantum computing services. With this growing accessibility, there arises a risk that malicious users may attempt to execute code on these systems to leak sensitive information from other users, disrupt or manipulate computational outcomes, or reverse engineer the underlying architecture and its associated intellectual property. To address these emerging security threats in cloud-based quantum environments, this chapter introduces and examines various forms of quantum computer virusesQuantum computer viruses. The study demonstrates that such malicious circuits can degrade the performance and correctness of quantum applications, including Grover’s search algorithmGrover’s search algorithm and quantum machine learningQuantum machine learning classification tasks. As a defense, this chapter presents a first-of-its-kind quantum computer antivirusQuantum antivirus designed to safeguard expensive and fragile quantum hardwareHardware-level quantum threats from such threats. The proposed antivirusQuantum antivirus inspects quantum programs, also referred to as circuits, prior to execution, enabling the detection of potentially harmful code. Implemented as a compile-time measure, the approach ensures zero additional overhead during the actual runtime of the quantum computer.