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In 1998 Robert Morris, a university student, unleashed a worm which affected 10 per cent of all the computers connected to the internet (at the time the net was estimated to consist of 60,000 computers), slowing them down to a halt. Morris is now an associate professor at MIT.

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The Concept virus, accidentally shipped on a CD-ROM supplied by Microsoft in 1995, was the first virus to infect Microsoft Word documents. Within days it became the most widespread virus the world had ever seen, taking advantage of the fact that computer users shared documents via email.

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The Chernobyl virus (also known as CIH) triggers on April 26 each year, the anniversary of the Chernobyl nuclear disaster. It overwrites a chip inside PCs effectively paralysing the entire computer. Its author, Chen Ing Hau, was caught by the authorities in Taiwan.

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The Anna Kournikova worm posed as a picture of the tennis player, but was in fact a virus written by Jan de Wit, an obsessed admirer from the Netherlands. He ended up receiving a community service sentence.

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The Melissa virus, written by David L Smith in homage to a Florida stripper, was the first successful email-aware virus and inserted a quote from The Simpsons into Word documents. Smith was later sentenced to jail for causing over $80 million worth of damage.

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Strengthen your passwords. Use random combinations of letters, numbers and special characters. Create different passwords for each account and change them frequently.

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Shred your documents. Don’t toss bank statements and credit card receipts in the trash. Dispose of them using a cross-cut shredder or shredding service.

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Guard your Social Security number. Avoid sharing it when it’s not absolutely necessary, and don’t keep it, or your Social Security card, in your wallet.

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Be smart about social media. Consider leaving personal details, such as your birthday or address, off your profiles. Strengthen your privacy settings and be cautious about whom you accept as a connection.

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. Secure your phone. Lock your device with a password, turn off Bluetooth when you’re not using it, and be cautious when downloading apps — especially free versions of popular apps, which may contain malware.

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Yahoo

Date: 2013-14
Impact: 3 billion user accounts
Details: In September 2016, the once dominant Internet giant, while in negotiations to sell itself to Verizon, announced it had been the victim of the biggest data breach in history, likely by “a state-sponsored actor,” in 2014. The attack compromised the real names, email addresses, dates of birth and telephone numbers of 500 million users. The company said the "vast majority" of the passwords involved had been hashed using the robust bcrypt algorithm.

A couple of months later, in December, it buried that earlier record with the disclosure that a breach in 2013, by a different group of hackers had compromised 1 billion accounts. Besides names, dates of birth, email addresses and passwords that were not as well protected as those involved in 2014, security questions and answers were also compromised. In October of 2017, Yahoo revised that estimate, saying that, in fact, all 3 billion user accounts had been compromised.

The breaches knocked an estimated $350 million off Yahoo’s sale price. Verizon eventually paid $4.48 billion for Yahoo’s core Internet business. The agreement called for the two companies to share regulatory and legal liabilities from the breaches. The sale did not include a reported investment in Alibaba Group Holding of $41.3 billion and an ownership interest in Yahoo Japan of $9.3 billion.

Yahoo, founded in 1994, had once been valued at $100 billion. After the sale, the company changed its name to Altaba, Inc.