Threats to Your Computer and Data: Malware, Spam and Firewalls

You download a “free” game from a website you have never heard of. The next day, your computer is slow, adverts keep popping up and your friends get strange messages from you. What happened, and how could you have stopped it?

In this post:

What malware is, and six common types: viruses, worms, Trojans, spyware, adware and ransomware
What spam is and why it is risky
How antivirus software and firewalls protect you
Simple habits that keep your data safe
A malware matcher, an inbox sorter, a firewall activity and a quiz

What is malware?

Malware is short for malicious software: programs written on purpose to harm a computer, spy on its user, or steal or lock up data. People often call all malware “viruses”, but a virus is just one type.

Type	How it works	Clues it might be there
Virus	Attaches itself to a normal file or program. It spreads when someone opens that file or shares it with others.	Files that suddenly will not open or are damaged; the computer crashes
Worm	Copies itself from computer to computer across a network, without anyone opening a file.	A whole network slows down; lots of computers have problems at once
Trojan horse	Pretends to be something useful, like a free game or app, but secretly does harm once you install it.	A new program you did not mean to install; settings change on their own
Spyware	Secretly watches what you do. A keylogger records every key you press, including passwords.	Accounts get logged into by someone else
Adware	Floods you with unwanted adverts and may change your browser’s home page.	Pop-ups everywhere; a strange search page
Ransomware	Locks your files (often by scrambling them with encryption) and demands money to unlock them.	A message saying your files are locked and you must pay

Remember the Trojan horse: in an old Greek story, soldiers hid inside a giant wooden horse that looked like a gift. The city pulled it inside, and at night the soldiers climbed out. Trojan malware works the same way: it looks like a gift but hides something harmful.

How does malware get in?

Opening email attachments or links from people you do not know
Downloading “free” or cracked games, movies and apps from unofficial websites
Plugging in a USB flash drive that has malware on it
Not installing updates, so old weaknesses stay open
Clicking pop-ups that say “Your computer is infected! Click here!”

Spam

Spam is unwanted junk messages sent to huge numbers of people at once, by email, text or social media. Most spam is advertising, but some carries malware or tries to trick you into giving away information (you will learn more about these tricks, called phishing, in the next post). Email services have spam filters that move most of it into a Spam or Junk folder. Never reply to spam, and do not click its links. Replying tells the sender your address is real.

Your defences

Defence	What it does
Antivirus (anti-malware) software	Scans files, downloads and USB drives for known malware. It quarantines (locks away) or deletes anything harmful. It must be updated often so it can recognise new malware. Windows includes one, called Microsoft Defender.
Firewall	Checks the data travelling between your computer or network and the internet. It lets safe traffic through and blocks traffic that breaks its rules, like a security guard at a gate. A firewall can be software on your computer or a separate device on a network.
Updates	Fix weaknesses in the operating system and apps that malware could use to get in.
Backups	Keep copies of important files on another drive or in the cloud, so even ransomware cannot take them away for good.
Smart habits	Download only from official app stores and websites, think before you click, and ask an adult if something seems wrong.

Two Windows screenshots. On the left, Windows Security lists a severe current threat called Ransom:Win32/WannaCrypt with a Start actions button. On the right, a message says a file could not be moved because it contains a virus or potentially unwanted software. — Microsoft Defender, the antivirus built into Windows, has found the WannaCrypt ransomware (left) and stops the file from being used (right). WannaCry, as it is often called, attacked hundreds of thousands of computers around the world in 2017.

Image source: File:Windows defender detects and block a ransomware from being executed.png (Wikimedia Commons)

A diagram with three computers on the left labelled LAN, a red brick wall in the middle labelled Firewall, and many computers on the right labelled WAN. — A firewall sits between a local network (LAN), such as a school’s, and the wider internet (WAN), checking the traffic that passes between them.

Image source: File:Firewall.png (Wikimedia Commons)

Watch: let’s learn about malware

This video from a security company’s series for children explains what malware is and how to stay safe from it.

Video: Safer Kids Online – Let’s learn about malware! – ESET North America – https://www.youtube.com/watch?v=eNBx9kbgdDY

Activity: name that malware

Try it: which type of malware is it?

Read each description and choose the type of malware.

A “free” game that secretly lets a stranger control your computer
Records every key you press and sends your passwords to someone else
Locks all your photos and demands money to unlock them
Spreads across a school network by itself, from computer to computer
Fills your screen with pop-up adverts
Hides inside a document and spreads when the file is shared and opened

Activity: sort the inbox

Try it: spam or genuine?

Drag (or tap) each message into the right box.

From your teacher’s school address: “Reminder: projects are due on Friday.”

“Your account will be closed. Download the attached file to fix it.”

“Cheap watches, 90% off, buy now!!!” from an address you do not know

“CONGRATULATIONS!!! You won a new phone! Click here to claim within 1 hour!”

“Hi, it’s your cousin. Here are the photos from Sunday’s lime.” (You were at the lime.)

From the school library: “The book you reserved is ready to collect.”

🗑️ Spam or risky

📥 Probably genuine

Activity: be the firewall

Try it: allow or block?

Your school’s firewall has two rules: allow normal web browsing and school email; block unknown programs trying to connect in from the internet, and known dangerous websites. What should it do with each connection?

A student opens the school’s homework website
An unknown computer overseas tries to connect to the office computers
A teacher sends an email through the school email system
A program tries to reach a website on the list of known malware sites
A student researches hurricanes on a well-known encyclopedia site
A strange program on a laptop keeps trying to send data out to an unknown address at night

Self-check quiz

Click the best answer. You get feedback straight away; only your first click counts towards your score.

1. Malware is…

2. Which malware spreads across a network all by itself?

3. Which malware pretends to be a useful program?

4. Spam is…

5. What does a firewall do?

6. Why must antivirus software be updated often?

Updates

2026/10/21

– Fixed the activity code so its buttons keep working.

Island Class

Learning in the Caribbean

Threats to Your Computer and Data: Malware, Spam and Firewalls