I would like to give an emphasis to
malware
What is malware?
Malware, or “malicious software,” is an umbrella term that describes any malicious program or code that is harmful to systems.
Hostile, intrusive, and intentionally nasty, malware seeks to invade, damage, or disable computers, computer systems, networks, tablets, and mobile devices, often by taking partial control over a device’s operations. Like the human flu, it interferes with normal functioning.
Malware is all about making money off you illicitly. Although malware cannot damage the physical hardware of systems or network equipment (with one known exception—see the Google Android section below), it can steal, encrypt, or delete your data, alter or hijack core computer functions, and spy on your computer activity without your knowledge or permission.
How can I tell if I have a malware infection?
Malware can reveal itself with many different aberrant behaviors. Here are a few telltale signs that you have malware on your system:
Your computer slows down. One of malware’s main effects is to reduce the speed of your operating system, whether you’re navigating the Internet or just using your local applications.
A tidal wave of annoying ads that shouldn’t be there washes over your screen. Unexpected pop-up ads are a typical sign of a malware infection. They’re especially associated with a form of malware known as adware. What’s more, pop-ups usually come packaged with other hidden malware threats. So if you see something akin to “CONGRATULATIONS, YOU’VE WON A FREE PSYCHIC READING!” in a pop-up, don’t click on it. Whatever free prize the ad promises, it will cost you plenty.
Your system repeatedly crashes, freezes, or displays a BSOD (Blue Screen of Death), which can occur on Windows systems after encountering a fatal error.
You notice a mysterious loss of disk space, probably due to a bloated malware squatter which hides in your hard drive.
There’s a weird increase in your system’s Internet activity.
Usage of your system resources is abnormally high and your computer’s fan starts whirling away at full speed—signs of malware activity taking up system resources in the background.
Your browser’s homepage changes without your permission. Similarly, links you click send you to an unwanted web destination. This usually means you clicked on that “congratulations” pop-up, which downloaded some unwanted software. Likewise, your browser might slow to a crawl.
New toolbars, extensions, or plugins unexpectedly populate your browser.
Your antivirus product stops working and you cannot update it, leaving you unprotected against the sneaky malware that disabled it.
Then there’s the painfully obvious, intentionally non-stealthy malware attack. This famously happens with ransomware, which announces itself, tells you it has your data, and demands a ransom to return your files.
Even if everything seems to be working just fine on your system, don’t get complacent, because no news isn’t necessarily good news. Powerful malware can hide deep in your computer, going about its dirty business without raising any red flags as it snags your passwords, steals sensitive files, or uses your PC to spread to other computers.
How do I get malware?
The recipe for a malware infection calls for a long list of ingredients. Topmost are the two most common ways that malware accesses your system—the Internet and email. So basically, anytime you’re connected online.
Malware can penetrate your computer when (deep breath now) you surf through hacked websites, click on game demos, download infected music files, install new toolbars from an unfamiliar provider, set up software from a dicey source, open a malicious email attachment, or pretty much everything else you download from the web onto a device that lacks a quality anti-malware security application.
Malicious apps can hide in seemingly legitimate applications, especially when they are downloaded from websites or messages instead of a secure app store. Here it’s important to look at the warning messages when installing applications, especially if they seek permission to access your email or other personal information.
For more details
https://www.malwarebytes.com/malware/ 
Fortunately I have been a very long time on the internet and didn't fall for it.
