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The World Health Organization had just declared COVID-19 a pandemic when intensive care units in the United States started to see an influx of severely ill patients. It was mid-March, and though coronavirus cases had been mounting in countries including China, South Korea and Italy, in the U.S. there was still a dearth of knowledge about how the virus spread, how it affected patients, and what type of threat it posed to the doctors treating them.

Within three months, critical care physicians across the country received a crash course on a disease that didn't exist in the U.S. before this year, and are more prepared in the event of a second wave of the illness. Now, in June, doctors have a better sense of which medicines and interventions to use or avoid, how the virus affects the body, and how to face their own COVID-19 fears.

In the beginning, "everyone had the concern of getting infected," Dr. Francis Castiller, medical director of critical care at UNC REX Hospital in Raleigh, North Carolina, said. The new disease was spreading rapidly, before many ICUs were able to prepare for the surge or protect their staff appropriately.

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Dr. Josh Denson, a pulmonary medicine and critical care physician in New Orleans, said he diagnosed the first critically ill COVID-19 patient in Louisiana. But the hospital did not yet have strict protocols for quarantining patients.

"They hadn't isolated this patient appropriately, so my team members and I were exposed," said Denson, who works at Tulane Medical Center but was at a different hospital when he was exposed to the virus. "We had real concerns about whether we were going to get this or not."

He never got sick, and has since tested negative for COVID-19 antibodies.

But it was those fears, in part, that affected how critically ill patients were cared for in the beginning of the outbreak in the U.S.

Ventilators

COVID-19 notoriously wreaks havoc on the lungs, leaving severely ill patients struggling to breathe. As cases started emerging in the U.S., doctors looked to their colleagues in Italy, who were already in the middle of a huge influx of extremely sick patients.

For patients with severe breathing problems, the Italian doctors were using a type of therapy called high flow nasal oxygen, a much less invasive approach than putting a patient on a mechanical ventilator. Patients can get 100 percent oxygen through the nose without having to have a breathing tube put in place.

But an unusually high number of health care personnel in Italy — 20 percent, according to an editorial in The Lancet medical journal — were becoming infected with the coronavirus. They blamed the high flow nasal oxygen, figuring the treatment was aerosolizing the virus, spreading it to doctors and nurses.

As a result, many doctors in the U.S. were initially wary of using high flow oxygen for COVID-19 patients.

"We were very concerned, so we didn't use it," said Dr. Hugh Cassiere, director of critical care medicine at Northwell Health's North Shore University Hospital on Long Island, New York.

Instead, patients were intubated and put on ventilators, often right away. "Reports from other places was that you should put people on the ventilator early, because the disease was so rapidly progressive," Dr. Todd Rice, an associate professor of medicine at Vanderbilt University Medical Center, said.

But putting patients on ventilators comes with risks, too, including infection and unintentional damage to the lungs. Very often, patients require heavy sedatives to paralyze them so doctors can get the breathing tube into the patients' windpipe. That procedure, called intubation, also carries the risk of infection and lung complications, and can expose health care workers to virus-filled respiratory droplets.

That's a big deal. If you can prevent someone from being intubated, that could change their whole course.

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What's more, the longer a person remains on a ventilator, the greater the chances for blood clots, gastrointestinal bleeding, pneumonia and death.

The first few months were a learning experience. Now, doctors are trying to avoid ventilators if possible. Both Rice and Cassiere said more current data show high flow oxygen does not put health care workers at increased risk. And experience has shown them that not all patients require a ventilator. When possible, doctors see if patients improve with the high flow oxygen first.

"That's a big deal," Cassiere said. "If you can prevent someone from being intubated, that could change their whole course."

Targeting the kidneys

Despite attempts to move away from ventilators, some COVID-19 patients still need them. As the pandemic has progressed, it's become apparent that coronavirus patients on ventilators need special care.

When patients are put on a ventilator, they're often given diuretics to get rid of extra fluid in the body. Lungs that need help need to be "dry" to function properly. When they're wet, "they can't move oxygen as well," Denson said.

But the coronavirus has since proved it's not a simple respiratory illness. It can affect the lungs, the brain, the blood and, critically for patients on ventilators, the kidneys.

Unlike lungs, kidneys prefer to be hydrated. The longer patients are kept dehydrated, their chances of kidney failure increase. Denson said he's changed his treatments for COVID-19 patients to give additional hydration if they're showing damage to the kidneys.

"I'm targeting the kidneys a little bit more," he said. "I'm less aggressive up front getting people dry, and I'm more willing to use fluids if needed."

It's a balancing act that requires extreme attention on the part of ICU doctors and their staff. Too much hydration hurts the lungs. Too little hurts the kidneys. "It's a constant battle," Denson said.

Medications

When doctors faced the first surge of severely ill COVID-19 patients, no drugs had been shown to work against the virus, making treatment more challenging. As a result, doctors were willing to try certain medications based on limited evidence.

Early on in the pandemic, the drug hydroxychloroquine emerged as a potential treatment, following two studies that suggested it might be beneficial. As a result, many patients were given the drug, which is already approved for malaria and rheumatoid arthritis. But doctors soon found the drug was not useful in treating COVID-19, and subsequentresearch has shown it does not appear to help.

Now, doctors in ICUs are turning to the drug remdesivir. It's not a cure, but it's the only treatment that's been shown in a clinical trial to have an effect on the illness so far.

Some physicians are also finding success with other pharmaceutical approaches, though evidence remains anecdotal.

Cassiere has given ventilated patients steroids to reduce inflammation in the lungs.

"I was gun-shy up front about doing that, because I was concerned I could be doing more harm," Cassiere said, citing research from the 2003 SARS outbreak that suggested steroids cause coronaviruses to linger longer in patients. He found that combining the steroids with convalescent plasma, an antibody-rich blood product of recovered COVID-19 patients, appeared to cancel out that risk.

Cassiere also said he's changed his methods of sedating patients who need to be put on a ventilator, opting for fewer narcotics like fentanyl in favor of other drugs such as benzodiazepines or ketamine.

"My experience has been that the narcotics hang around longer, and may have something to do with the prolonged awakening some of these patients have," Cassiere said, referring to those who take an unusually long time to wake up from a coma after being removed from a ventilator.

Maintaining connection through a lengthy illness

That COVID-19 patients tend to be sick for a long time, spending weeks in the intensive care unit in some cases, is another factor physicians are getting used to in dealing with COVID-19.

"Taking care of patients requires a lot of patience," Dr. Steve Stigler, director of the medical intensive care unit at the University of Alabama at Birmingham, said. He coaches his physicians to stay the course with treatment and supportive care.

Patients "improve up to a point, and then it can be several weeks before we would see them continue to improve," Stigler said.

Castiller, of UNC REX Hospital in Raleigh, also said it's critical for ICU physicians to communicate that to families of COVID-19 patients.

One of the biggest lessons we've learned is the importance of human contact.

"Families need to prepare for that, as well as peaks and valleys" seen so often in the sickest patients, Castiller told NBC News. To offer support, Castiller said his staff calls patients' families daily with updates.

Rice's team at Vanderbilt does the same. "Every day, we call families and say, 'Here's the update on your loved one.' It's gone really, really well, and it's something we're proud of," he said. The staff also uses videoconference technology, like Zoom, so the family can visit with patients.

"One of the biggest lessons we've learned is the importance of human contact," Castiller said. Hospital restrictions that prohibit visiting COVID-19 patients have been major stressors for families, as well as those in the hospital. "We make sure to address that by using technology to maintain some level of communication."

'I'm a COVID-19 warrior now'

Early fears that critical care physicians had about becoming infected with the coronavirus have eased significantly. Cassiere, who said he was terrified at the beginning of bringing the virus home to his family, has tested negative for antibodies. He credits appropriate use of personal protective equipment, such as masks, gloves and gowns.

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"All my protection I've had has helped. I'm confident that I'm not bringing it home. I'm confident that if I'm protected, I'm not going to get infected," Cassiere said. "And now, I'm armed with the knowledge and different approaches I have for battling COVID-19."

Experience matters. "Being a good critical care doctor is a lot of experience," Rice added. "Now we've seen this, and we've done this, and it will result in us providing even better care for our patients."

"We know we don't know everything about it, but we know the spectrum of disease and what it does to the body," Cassiere said. "I didn't know that back in March. I have all that knowledge behind me. I'm a COVID-19 warrior now. We're totally prepared for it."

Erika Edwards is a health and medical news writer and reporter for NBC News and "TODAY."