By Monday afternoon, it the ransomware had targeted around 200,000 organisations in 150 countries.   

The "WannaCry" ransomware appears to have used a flaw in Microsoft's software, discovered by the National Security Agency and leaked by hackers, to spread rapidly across networks locking away files. 

Ransomware is a kind of cyber attack that involves hackers taking control of a computer or mobile device and demanding payment.

The attackers download malicious software onto a device and then use it to encrypt the victim's information. They threaten to block access to the files until a ransom is paid. It is common for criminals to ask for a fee between 0.3 and 1 Bitcoins (£400 - 1,375). 

Such attacks are mostly waged against businesses, but can also affect individuals. 

Here are some ways to protect yourself from ransomware. 

Back up your files 

The greatest damage people suffer from a ransomware attack is the loss of files, including pictures and documents. 

The best protection against ransomware is to back up all of the information and files on your devices in a completely separate system. A good place to do this is on an external hard drive that isn't connected to the internet. This means that if you suffer an attack you won't lose any information to the hackers.

Businesses often save copies of their data to external servers that won't be affected if their main network is attacked. 

Be suspicious of emails, websites and apps

For ransomware to work hackers need to download malicious software onto a victims computer. This is then used to launch the attack and encrypt files. 

The most common ways for the software to be installed on a victim's device is through phishing emails, malicious adverts on websites, and questionable apps and programs. 

People should always exercise caution when opening unsolicited emails or visiting websites they are unfamiliar with. Never download an app that hasn't been verified by an official store, and read reviews before installing programs. 

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Use an antivirus program 

An age-old computer security tip, antivirus programs can stop ransomware from being downloaded onto computers and can find it when it is.

Most antivirus programs can scan files to see if they might contain ransomware before downloading them. They can block secret installations from malicious adverts when you're browsing the web, and look for malware that may already be on a computer or device. 

Always install updates 

Companies often release software updates to fix vulnerabilities that can be exploited to install ransomware. It is therefore advisable to always download the newest version of a software as soon as it is available. 

Never pay the ransom

Victims of ransomware attacks are advised to never pay the fee as it encourages attackers and may not result in files being recovered. There are some programs that can help decrypt files. Or, if you have a back up, you can restore your device from that.