These are the type of programs, we generally download onto our computer…

Concept: Malware is classified along two independent axes: how it gets onto a system (a user is tricked into downloading/running a disguised program, versus it propagates on its own) and whether it self-replicates by copying itself into other files or across a network.

Application: The stem describes a program the user themselves downloads because it looks useful or free, which then seizes control of the system and damages data. That is a deception-based, user-installed delivery with no mention of copying itself into other files or spreading across a network on its own — exactly the delivery-plus-behaviour profile of a Trojan (Trojan Horse).

• Virus: needs an existing file or program to attach to, and only activates and copies itself into other files when that host file is run — it cannot exist as a stand-alone downloaded program.

• Worm: spreads itself automatically across a network from machine to machine without any user choosing to download or run it.

• Spam: is unsolicited bulk messaging, not executable software at all, so it cannot take control of a system or damage data.

Result: Only a Trojan matches both the deceptive, user-driven download and the take-control-and-damage behaviour, so Trojan is correct.