The smartphone has become one of the most important tools in millions of Americans’ lives. It tracks your movements, displays emails and text messages, and notifies you of every birthday and appointment. Every second, information floods your smartphone. Unless you switch them off, your apps work round-the-clock, obeying your every setting and preference.

Your phone churns private data through its circuitry all day long, and if criminals can break into it, they can steal all kinds of things, from banking details to compromising photos and videos. And they don’t actually have to steal your phone. They may not even be located in your country.

How do they do it? Spyware.

What is spyware?

Spyware is kind of like a computer virus, except instead of messing up your hard drive, it enables strangers to snoop on you. Skilled hackers can install spyware on your phone without you even realizing it.

Click here to learn how to spot five spyware apps that could be watching and listening to you right now.

Once spyware is on your phone, it can record everything you do, from sending text messages to shooting video of your family reunion. Hackers may use it to break into your private accounts, commandeer your email and even blackmail you.

Keep in mind, “spyware” is a vague and multifaceted term. It’s not always malevolent. Some parents install a kind of spyware on their kids’ smartphones in order to keep track of their activities. Managers sometimes keep tabs on their employees by watching what they do on their company computers. I don’t endorse this — I think there are much healthier ways of watching kids and employees — but this kind of spyware isn’t intended to ruin your life.

Don’t click on strange links

The easiest way to avoid downloading spyware is this: Don’t click on strange links. If you receive an email from a suspicious stranger, don’t open it. If you receive an email or text from someone you do know, but the message seems peculiar, contact your friend by phone or social media to see if the message was intended.

This might sound obvious, but sometimes our curiosity gets the better of us. When a link appears, some of us struggle to avoid clicking, just because we want to know where it leads. Other times, an authentic-looking email is actually a phishing scam in disguise. If you’re the least bit doubtful, don’t click.

Lock your phone

Though some phones are more susceptible to spyware than others (more on this later), owners can dramatically reduce their chances of infection by locking them. A simple PIN number will deter most hackers.

Also, avoid lending your phone to strangers. Yes, some people honestly forget their chargers at home and urgently need to call their spouses, but a clever con artist needs your unlocked phone for only a minute to cause a lot of damage. In this case, being a Good Samaritan is risky business.

Androids and spyware

The bad news is this: Android phones are particularly vulnerable to spyware. It’s simple to install a spying app on any Android gadget, but only once you get past the lock screen.

To protect yourself, make sure you have the lock screen turned on and that no one knows your PIN, password or pattern. You can make it even harder by blocking the installation of third-party apps. To do this, go to Settings >> Security and uncheck the Unknown Sources option. It won’t stop a really knowledgeable snoop, but it could stump less-savvy ones.

iPhones and spyware

Apple users can get pretty smarmy about their products. If you own an iPhone, you probably already know that your phone is far safer from malware than Android gadgets. A recent “Forbes” study showed that nearly 97 percent of all known malware threats affect only Android devices.

That’s good news for Apple addicts, but it can also make them overconfident.

Last August, Apple had to release an extremely critical iOS update to patch a security threat. Before the update, an attacker could take over and fully control an iPhone remotely just by clicking the right link.

The spyware was called Pegasus and the type of attack was called Trident. The latest iOS was partly designed to prevent these exploits from damaging your iPhone. This is just one reason you should keep your iPhone up to date.

To get the latest version of iOS, go to Settings >> General >> Software Update. Your device will then check for the latest version of the Apple iPhone and iPad operating system.

Windows smartphones

At the moment, Windows Phone 8 and Windows Mobile 10 seem to be fairly well protected against mainstream spyware apps. Don’t rest on your laurels, though; an unlocked Windows phone is still inviting disaster.

Secondhand smartphones

Beware secondhand smartphones. Sometimes they’re handy, because jailbroken phones are cheap and disposable and may work with many service providers. But they may also come with spyware installed.

Buying a secondhand phone is a common practice, especially if you’re traveling in a foreign country or you’re between contracts and need something just for the short term.

If you have any suspicions about your phone, your best tactic is to reset factory settings. It’s inconvenient, but it might save you a lot of heartache down the line.

What about your headphones? Could they be listening too? What about billboards? Click here for three things that could be spying on you right now.

What questions do you have? Call my national radio show. Click here to find it on your local radio station. You can listen to the Kim Komando Show on your phone, tablet or computer. From buying advice to digital life issues, click here for my free podcasts.

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Learn about all the latest technology on the Kim Komando Show, the nation's largest weekend radio talk show. Kim takes calls and dispenses advice on today's digital lifestyle, from smartphones and tablets to online privacy and data hacks. For her daily tips, free newsletters and more, visit her website at Komando.com.