It’s all over the news: Charlie Sheen is HIV-positive. However, during his big reveal on the Today Show, his physician, Dr. Robert Huizenga, mentioned that Sheen’s viral load was “undetectable.”

To make matters more complicated, Charlie Sheen told the Today Show that “it’s impossible for him to transmit the virus to somebody else.” (He later clarified that he meant through protected sex).

Still, nothing is foolproof, and his ex-girlfriend, Bree Olson, told Howard Stern that they had unprotected sex together after he was diagnosed. In fact, she claims Sheen never even told her he was HIV-positive.

This raises a lot of questions—namely, is he infectious, and what does it mean to have an undetectable viral load in the first place?

Typically, when somebody is first infected with HIV, they have around a million copies of the virus in a ml of blood (or a 1/5 of a teaspoon). Once their immune system gets to work, that number naturally decreases to about 5,000 to 50,000 (without any medication).

When someone has an “undetectable level of HIV,” that number shrinks drastically—to just under 20 copies of the virus in the same amount of blood, Cornell University Professor of Medicine and HIV researcher Dr. Jeffrey C. Laurence, told Upvoted.

Usually a patient would have to take a strict regimen of HIV medication (called antiretroviral therapy or ART) to keep those numbers low. Keep in mind, though, this won’t eliminate HIV altogether.

Sometimes, a patient’s number of copies will strangely drop to undetectable levels without any medication at all. These bizarre patients are called “elite controllers,” and, for obvious reasons, they embody the holy grail of HIV research. There’s something special about their immune system, says Laurence.

So how likely is it that Sheen infected someone if he was having unprotected sex? Not likely, if his levels are truly undetectable. According to Laurence, it’s difficult to transmit the virus if your viral load is under 1,000. Though, it is certainly still possible.

Laurence’s explanation is corroborated by a 2014 study that analyzed the sexual habits of over 1,000 couples—each couple was comprised of someone who was HIV-positive (but actively taking ART medication) and someone who was not.

Incredibly, the couples were required to have sex without a condom in order to participate.

After a period of two years, researchers found that none of the partners had been infected by the virus. And this wasn’t an inclusive study by any means. Around 40 percent of all couples were homosexual. And most couples—gay or straight—engaged in all sorts of sexual acts, including anal sex.

Still, Laurence recommends patients with an undetectable viral load to practice safe sex and to disclose that they have HIV to their partners.

In fact, it’s illegal not to disclose HIV status to sexual partners in much of the United States. In California, where Sheen resides, the offense is sometimes a felony. In some cases, it’s an offense punishable by up to eight years in jail.

There’s also an issue when someone says they are cured—because, simply, there is no cure. Consider the famous case of former basketball star Magic Johnson.

When he became undetectable, just like Sheen, Johnson’s wife announced to the world that her prayers had cured him, says Laurence. This sparked a bunch of rumors—still persisting to this day—that Johnson had eradicated the disease.

“It’s a very unfortunate statement. She took ‘undetectable’ to mean it cleared the virus from his body. And that’s not true. The virus is still there. He’s potentially still infectious,” said Laurence.

In fact, Johnson (and Sheen) can still become highly infectious when they experience “viral blips” on random days. When an undetectable person’s immune system is comprised—even by something relatively harmless like a bad case of the cold—they can become “detectable” again.

“You never know,” said Laurence.

All in all, ART really does work wonders. If Sheen continues to take his pills everyday, he can expect to have a normal life expectancy. Of course, his life is not going to be without problems (there are severe side effects that can lead to heart disease and stroke).

“I was very impressed with the Charlie Sheen interview… The fact that a celebrity who has the disease is not ashamed, helps other people get tested,” shares Laurence.