An alarming report from federal health officials found that drug-resistant "superbugs" are infecting millions of Americans and killing thousands each year.

Reigning in a "post-antibiotic era," the Centers for Disease Control and Prevention found that 2.8 million people are infected and more than 35,000 people die every year from the bugs.

Superbugs are hard to treat bacteria and fungi that have developed resistances to common drugs, like antibiotics or antifungals, that had been used to treat them.

"Stop referring to a coming post-antibiotic era – it’s already here," CDC Director Robert Redfield says in a letter accompanying the report.

Drug-resistant bugs are in every U.S. state and around the globe, and they now cause nearly twice as many deaths as they did when the CDC reported on it in 2013.

Here's what you need to know about superbugs:

What is a superbug and is there a treatment?

Superbugs are bacteria or fungi that are resistant to antibiotics or antifungals, says Dr. Alyssa Letourneau, director of the Massachusetts General Hospital Antimicrobial Stewardship Program.

Antibiotics can be very narrow and target one specific bacteria, or they can have a broad range of the bacteria they fight.

'Post-antibiotic era' is here: Drug-resistant superbugs sicken 2.8M, kill 35K each year

When a patient comes in with a bacterial infection, a doctor may prescribe a drug that targets the specific "bad" bacteria causing the illness. These antibiotics, in theory, will be better at eliminating the bacteria causing the infection while not harming the "good" bacteria in a patient's system.

But bacteria are smart, Letourneau says, and they've been around for millions of years, so they can over time mutate to become resistant to the drug that fights them.

For example, if a child has many ear infections one year, she says, the next year the penicillin used to treat them may not be as effective.

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"If we take an antibiotic and we put it with bacterium, it's pretty easy to select resistance. It's really a natural process," says Dr. Robin Patel, president of American Society for Microbiology and a doctor at the Mayo Clinic.

This process can happen in a matter of days.

If a bug resists the narrow-spectrum drug used to treat it, doctors can use broad-spectrum antibiotics. However, doctors ideally stay away from prescribing these antibiotics if unnecessary because they can target and kill more bacteria – including the good ones not causing harm, Letourneau says.

Those antibiotics "can be more toxic, can be more expensive, and also some times may not work," she says.

Using these antibiotics can expose more bacteria to the drug and then in turn allow them to become resistant, even if they weren't causing an infection in the first place, Patel says.

Over time, bacteria can become resistant to all types of antibiotics.

"Those are the ones that keep me up at night and that I worry about," Letourneau says.

What are the superbugs the CDC warned about?

The CDC identified 18 bacteria and fungi for health officials to monitor, including five described as "urgent threats."

Among the urgent threats is Candida auris, a drug-resistant fungus that began spreading among hospital and nursing home patients in the United States in 2015.

The urgent threats also included: Carbapenem-resistant acinetobacter; carbapenem-resistant enterobacteriaceae (CRE); drug-resistant Neisseria gonorrhoeae; and clostridioides difficile, or C. diff.

More: ‘Nightmare’ bacteria, resistant to almost every drug, stalk US hospitals

How do superbugs spread?

As bacteria and fungi are exposed to more drugs over time, they evolve and get better at staving off the antibiotics and antifungals from killing them.

According to the CDC report, the DNA in a bacterium or fungus can tell it to create specific proteins that drugs cannot break down, making that bacterium or fungus resistant to to the drug, which often targets proteins to kill it.

Bacteria can also share their drug resistance with other bacteria, Patel said. This involved sharing of genetic information between closely related bugs.

These resistances can develop within the bacteria people have in their bodies, and the bacteria can then spread from person to person, via animals and food or through the environment.

"It is an individual issue that we have to deal with but it's also a community issue and it's really a global issue," Patel said.

Additionally, otherwise healthy people may have a drug-resistant bacteria in their system, it just may not be causing an infection, Letourneau said.

Are symptoms of a superbug infection different and how do we treat them?

No. Symptoms of a superbug infection are not always the same and they usually look like any other infection, Letourneau says.

People don't necessarily know they have a superbug, she says. They come in with a set of symptoms and the doctor must figure out their risk for a superbug.

Doctors must look through a person's medical history and travel records when determining if the person's bug will be resistant to an antibiotic.

Antibiotic-resistant bugs are more common in countries that prescribe antibiotics more readily, Letourneau says.

Diagnosing which drugs will work against a superbug requires laboratory testing. When a patient comes in with a set of symptoms, they may be prescribed an antibiotic but it may take days of tests to identify a drug that will work, Letourneau says.

The challenge is an issue of time, she says. The sooner a patient starts on a treatment, the better their chances are. But starting a patient on a broad-spectrum drug when they may not need it can contribute unnecessarily to antibiotic resistance, Letourneau says.

It's a challenge to balance treating individual patients while also thinking about public health. But medical professionals have gotten better at diagnosing and identifying drugs that patients are not yet resistant to, Patel says.

How do we treat superbugs resistant to all drugs?

These bacteria and fungi are the ones doctors worry most about given how hard treating them can be and how quickly they're increasing in frequency, Letourneau says.

The first step is ensuring that it really is a drug-resistant bug that's causing the infection, Patel says. If there isn't any antibiotic that will work, doctors will turn to non-drug interventions. Surgery for an abscess with a deep infection, for example, can be better than an antibiotic in some cases, Patel says

There are also a number of new treatment options being developed still in their early stages. One option could be to use a virus to infect and kill a bacteria, Letourneau says.

In other cases, if a person has been given a broad-spectrum antibiotic that kills all the "good" bacteria in their gut, a stool transplant from someone with the right bacterial mix could provide enough of those bacteria to fight off the "bad" bacteria, Letourneau says.

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Should I still take antibiotics if I'm sick?

Antibiotics and antifungals are extremely effective in fighting off infections, and some antibiotics do still effectively treat superbugs not resistant to all drugs.

Antibiotics are "life-saving" drugs, and there's a reason they had been used for widely for so long. But, Patel says, "in the past we tended to think of antibiotics as fairly harmless drugs, and we need to reconsider that."

If you are prescribed an antibiotic by a doctor, take it as directed, Patel says. Do not self-prescribe antibiotics when you feel sick in a similar way, Patel adds.

Doctors ideally weigh whether an antibiotic is needed to treat a patient, but patients should feel empowered to ask their health care providers why an antibiotic is needed, Letourneau says.

In some cases, a virus may be causing an infection, and taking an antibiotic would not treat it. While cases of doctors prescribing antibiotics when unnecessary are decreasing, it does still occur and can contribute to drug-resistance, Letourneau says.

How do I prevent against getting a superbug?

Basic hygiene and prevention methods that have been around for years are still advised to protect yourself.

Wash your hands with soap and water. Be careful handling food or when around someone who may be sick.

In some cases, vaccines may have been developed for bugs resistant to drugs, Patel says.

Contributing: Ken Alltucker, USA TODAY. Follow USA TODAY's Ryan Miller on Twitter @RyanW_Miller

This article originally appeared on USA TODAY: Antibiotic resistant superbugs killing thousands: What to know