As of writing this, all infosec eyes are on what will happen with the WannaCry ransomware worm.
Will it continue to spread, and who will get it next? Will the criminals who wrote the worm alter their code to get around the "kill switch" an anonymous British security researcher found, and which slowed down the spread of WannaCry?
Will copycats snag the code and make something even worse than WannaCry? Actually, the answer to that is "yes"; it's almost certain there will be even nastier malware.
WannaCry is as bad as it is due to a combination of factors: it's destructive and encrypts your files like past ransomware, but it also replicates itself and spreads very fast on networks, looking for vulnerable Windows computers.
Those computers are often found in cash-strapped organisations like health authorities where WannaCry has locked doctors out of patient files already. Then there's vulnerable embedded systems in diagnostics and treatment systems, which too could be shut down by WannaCry.
Small-to-medium-sized businesses are also in WannaCry's crosshairs so to speak, as they too try make the most of old systems and don't update or upgrade these.
In both cases, a WannaCry attack could be devastating, cause firms to fold and even loss of life.
Protecting against WannaCry and other malware wannabes
Update all your systems frequently - this includes not just your computers, but network gear, routers, internet gateways, smartphones, everything that you use.
Don't hope for the best and use old gear. Once tech is past its use by date, it'll be dangerous.
Back up often, and make sure everything important is included. Make sure that you keep good copies of your data, and don't accidentally overwrite it with bad stuff... like ransomware encrypted files.
Simplify your IT. If you don't know what some piece of hardware or software does, or if it's not needed, out it goes. One less thing to worry about.
Use a hardware or software firewall, to control what you send to, and receive from, the internet.
Be careful where you go on the internet. There are some very dark and dangerous corners on it, where you will be mugged. Likewise, don't open emailed or sent via another method willy-nilly. Use cloud storage with anti-virus scanning abilities to share files.