A covid-fighting tool is buried in your phone. Turn it on.

But exposure notifications only work if you and the people around you turn them on. Yes, you!

There’s early evidence this anonymous smartphone technology works — but so far isn’t helping very many Americans. In August, I wrote about the first of these state-sponsored alerts, Virginia’s Covidwise app. In the three months that followed, only 488 people have used the state’s app to send alerts about a positive diagnosis to others.

The alerts use software built by Apple and Google into iPhones and Android devices to detect when people (or the phones they’re holding) get into close contact with each other. That might sound like a privacy invasion, but they figured out how to track encounters between people in a way that’s anonymous — and doesn’t store your location — by using the Bluetooth wireless technology in phones. With an update to iOS on Dec. 14, it now works on handsets as old as the iPhone 6.

“This isn’t going to be a silver bullet, but it’s one more tool in the toolkit,” said Michael Wilkening, special advisor to the California governor for innovation and digital services. It helps fill the gaps left by traditional contact tracing, which requires patients to recall where they’ve been and know who they might have exposed.

Exposure alerts worked for the governor of Virginia, Ralph Northam. He and the first lady tested positive for the coronavirus in September, and because they had it working on their phones, staff members exposed to them got notified. And they’re picking up steam: In its first week in December, California’s system was activated 6.5 million times, reaching about 16 percent of its population. More than 68 percent of Washington, D.C. has opted in as of Dec. 14.

So why aren’t our phones a big part of America’s coronavirus response? For starters, each state’s local health department has to develop and operate its own system (though they’ve recently begun making them work across borders). Privacy concerns about similar-sounding — but actually very different — contact-tracing apps have needlessly scared people away. And frankly, Apple and Google buried the settings and apps you’ll need, bungling what could have been the year’s most-helpful tech launch.

You don’t have much to lose, so you might as well turn exposure alerts on. It takes less than five minutes to set up, and this guide will help.

As of Dec. 24, 20 U.S. states and territories, plus D.C., support coronavirus exposure alerts. For instructions, click on the link next to each region — that will take you to a website made by local health authorities:

And these states have either announced their intent to launch services or are running limited tests:

Nations around the world are also using similar Bluetooth technology for notifications, including Switzerland, Ireland, Northern Ireland, Scotland and England.

Listen out for announcements from your local health department. I will also update the list above as more come online.

If you have an iPhone, this might also help: Go to Settings, then Exposure Notifications and make sure Availability Alerts are turned on.

States can use this system to push out notification to your phone when their systems go live. (This requires you to have location services turned on, so your phone can tell where you are.)

That depends on where you live and what kind of phone you have.

Most states have made their own iPhone and Android apps that walk you through the steps of turning on alerts and — should it be needed — entering in your own positive test result.

For iPhones only: In some places, including California, Colorado, Maryland, Washington state and D.C., there’s no app — you just need to activate it in your phone’s settings, under Exposure Notifications. You might even get a push alert on your phone suggesting you turn it on. (Note this won’t work for all states — you may still be required to download a separate app.)

You set it and forget it until you, or someone you’ve been around, gets a positive coronavirus test. That’s all you really need to know. Every once in a while you might get a notification just to remind you the system is still running.

Behind the scenes, your phone is constantly sending out little Bluetooth chirps. There’s not actually a sound, but the chirps contain an anonymous, constantly changing code that can be picked up by other phones you come near. The signals are also used to approximately measure distance. The stronger the Bluetooth chirp, the closer two phones are to each other.

Your phone is also listening for the chirps coming from other phones, keeping a rolling 14-day log of all the phones you come near.

You do have to leave your phone and your Bluetooth turned on, at least when you’re around other people. Don’t worry: In my tests, all this chirping and listening isn’t much of an additional drain on your battery. (If you still want to turn off your Bluetooth when you’re back in isolation at home, that won’t hurt — but you need to remember to turn Bluetooth back on when you go back outside.)

If you’ve had the alerts turned on and end up getting a positive test, you should tell your local health department. The authorities will give you a unique code, or key, to enter into your phone.

Entering the code sends out a kind of bat signal, letting other phones know to check if, and how long, you were in contact.

If there was close contact — usually within six feet for 15 minutes or more — then the phone will pop up an alert recommending a course of action, including getting a test. (Some systems, such as one being tested in Arizona, will let you know if you’ve had as little as five minutes of exposure.)

I’m usually the first person to caution that we shouldn’t trust corporations or the government with our sensitive personal data. But after investigating the data flowing out of these state-sponsored apps and services, I haven’t found much danger in having them on my phone.

Here’s why: These systems don’t log your phone’s location. Instead, they use the clever Bluetooth system that helps phones remember whom you were near without knowing where you were.

The person receiving an alert doesn’t know who they were exposed to, or even when exactly it happened. Even government health authorities don’t learn that (which some of them aren’t happy about, because it would help them with contact tracing). It’s all anonymous.

Exposure alerts also won’t be turned on without your permission. The software that makes them work is part of recent upgrades to iOS and Android, but it won’t activate until you tell it to.

It won’t hurt to use a different state’s alert system, but it might not help, either. You won’t get much benefit from the exposure alerts unless the people around you also have them turned on. And if your actual local health department doesn’t support the system yet, you’ll have no way to report your own positive diagnosis to warn others. (Doing that requires a special code from your health department.)

You’ll need a phone that can run recent software updates from Apple and Google that enable coronavirus notifications.

For Apple iPhones: The iPhone 6 (from 2014) and newer will work, so long as it is running at least iOS 12.5.

For Android phones: You’ll need Android version 6, which is supported on phones dating back to the Samsung Galaxy S5 (from 2014).

Bluetooth signals are not a perfect way to measure exposure. They can roughly measure distance, but we know your actual exposure risk changes a lot depending on whether you are indoors or outdoors, and whether you and the person you’re exposed to are wearing masks.

But so far, apartment living hasn’t proved to be a major problem for this technology. Bluetooth signals can pass through walls but don’t do it very well. (Just try listening with your Bluetooth headphones across the other side of a wall.) The systems have been tuned to emphasize fewer false-positive readings — meaning if you get an alert, chances are you have been very close to someone with a coronavirus infection for a long time.

No problem. Just make sure you’ve activated the alerts from the state or territory where you’d take a coronavirus test yourself. That state’s health department would be the one to provide you with the codes to enter a positive test result into the system.

There’s no need to install apps from multiple states. When the exposure apps first launched, they were all independent islands. But now they’re cooperating across state lines with a national repository of those positive-diagnosis keys. That means, if you live in New York and were exposed to someone from New Jersey, your phone would still be able to get the alert.

Citizen, a popular security app, released its own exposure-tracking system called SafePass that works nationwide. But unlike the other exposure notifications I’ve described here, it isn’t being run by local governments and requires people to accurately self-report their own coronavirus test results. Citizen says its system has been activated by more than 1.1 million people. But in the past 30 days, it has collected 130 positive diagnoses and sent only about 200 exposure notifications.

The technology has now been used by thousands of Americans who tested positive for the coronavirus to let others know they might have been exposed. What we don’t know is how many people have received those notifications. That number could be very small.

Virginia, which has been running its Covidwise app since August, has the most experience. As of Nov. 11, Virginia had more than 770,000 participants — a bit under 10 percent of the state’s population. Of those, 777 people with the state’s app installed had gotten a positive test, and just 488 of them had entered their diagnosis in the system to make alerts go out to others. Virginia says it doesn’t know how many people have gotten alerts that they were potentially exposed.

“Virginia is pleased with the continued upward trend in downloads,” said spokesman Jeff Stover. “However, we desire to have the majority of the population with the app on their devices.”

Pennsylvania’s app was downloaded half a million times in its first nearly two months. “We do believe Bluetooth exposure notification apps are effective,” said Maggi Mumma, deputy press secretary at the state’s Department of Health.

The Centers for Disease Control and Prevention says it is supporting research into the effectiveness of phone exposure notifications. “We are doing this work to help states reach conclusions and make decisions,” said spokesman Jason McDonald.

Other nations have reported clearer success. In Northern Ireland, as of the end of October, more than 16,000 people had received notifications to quarantine after nearly 5,000 positive test results were entered into its app, according to the BBC.

In Switzerland, the first nation to use this Bluetooth notification system, academic researchers say early data on people testing positive after receiving notifications suggests the tech is making “relevant contributions to pandemic mitigation.”

This is new technology, and there’s a lot that engineers and health authorities are figuring out as they go along.

To start with, smartphones still aren’t universal. California estimates only about 70 percent of the state’s 39.6 million people could be eligible to participate.

The piecemeal rollout embodies many of the challenges in America’s coronavirus response. Asking each state to separately develop its technology has been a hindrance at a time when health departments are strapped for time, money, tech and marketing expertise. Apple and Google developed the underlying technology free, but many states still have struggled to make their own apps. A newer update, called the “express” system, allows states to roll out the technology with less effort. (The state of New York launched its system for less than $700,000.)

Engineers are still working, too, on figuring out how to best read the Bluetooth signals. To start, they were tuned to minimize potential false positives. But now they’re also learning about how the signals respond differently when people are indoors, outdoors — and have their phones in their pockets in either place.

“We are working as a community to optimize it and to figure out how to get those settings to be in the right place so that we do balance the risk of false positives with the getting notifications out to people who are at risk,” says Jenny Wanger of the Linux Foundation Public Health, which has been helping coordinate and provide tech to governments.

A public health challenge is also a marketing challenge. Lots of people have to use this technology for it to work, or even to see what needs to be tweaked to make it work better. Apple and Google have been helping states market it behind the scenes but haven’t included it in their own fancy product launches and TV commercials (including the ones dubiously promoting new smartwatches as “the future of health”). If they’re serious about this tech saving lives, they really ought to step up.

Even if participation is limited, you could still benefit from activating exposure alerts. Oxford University researchers say they think the alerts can be helpful at all levels of uptake. Participation by as little as 15 percent of a population, they said, could reduce infections and save lives.