Answer: A virus is triggered by an event on the host system.
Explanation: Virus malware is the defined as the malicious software that carries a virus like Trojan, etc.This is the event that takes place in the operating system as malware infection to steal the confidential data or attacking the system.
The virus gets invoked through some activity occurring in the host of the system.The virus can spread in the system if the malware program is executed.Thus, the source for the virus triggering is from the host.