What’s the Difference Between a Virus, Worm, and Trojan?

You hear these terms thrown around all the time:

“My computer has a virus!”
“Is that a worm?”
“I think it’s a Trojan.”

But are they all the same?
Not quite.

Let’s break down the three most well-known types of malware — and what makes each one unique (and dangerous).

A virus is a piece of malicious code that attaches itself to a legitimate file or program.

It needs user interaction to spread — like opening an infected file or running a program.

Once activated, it can:

✅ Think of it like a parasite — it needs a host to live and spread.

A worm is like a virus, but smarter — it doesn’t need a host file or user action to spread.

It replicates across networks, often using vulnerabilities in the system or shared drives.

What it does:

✅ Think of it as a digital infection that can “crawl” from one device to another.

A Trojan horse pretends to be something useful — a free game, a cracked app, a fake installer.

You run it, thinking it’s safe, and it secretly:

It doesn’t self-replicate like a virus or worm. It needs your permission to run — that’s the trick.

✅ Like the myth it’s named after, the real danger is inside the package.

Feature	Virus	Worm	Trojan
Needs user action?	✅ Yes	❌ No	✅ Yes
Self-replicates?	✅ Yes	✅ Yes	❌ No
Spreads via network?	❌ Sometimes	✅ Always	❌ No
Disguised as legit?	❌ Rarely	❌ Rarely	✅ Always
Damage level	Medium–High	High	Varies (Stealth)