For a child who is not vaccinated against measles — one of the world’s most infectious viruses — no classroom, school bus or grocery store is safe. Nine out of 10 unvaccinated people exposed to an infected person will catch it, and once measles takes root, the virus can damage the lungs, kidneys and the brain.

With falling U.S. vaccination rates and outbreaks that have caused more than 580 cases and at least one death, health experts expect measles to infect hundreds or even thousands more across the nation. Here is how measles takes over the body.

Unlike viruses that require person-to-person contact, measles lingers in the air for up to two hours after the person carrying it has left the space.

A child can inhale virus-containing droplets in a room where another child — unknowingly infected with measles — has been studying or playing an hour earlier. The virus can enter her body through the lining of her nose or mouth, or when she’s rubbing her eyes.

During the subsequent 24 hours, the virus takes root by lodging in her nasopharynx cells in the upper part of the throat and starts spreading to her lungs.

Then, the virus takes over, multiplying inside the cells and building up an army for an attack.

Within a few days, the virus begins to spread to infect the nearby lymphoid tissues. About a week after the initial exposure, infected cells begin traveling to other organs throughout the body. (At this time, the immune system of a vaccinated child would recognize the virus and fight it off.)

Typically, during the replication and spread of the virus, the child does not feel sick. The average incubation period is about two weeks — though it can range from one to three. When the viral load has increased significantly, it moves to infect other cells of the lungs and eyes, making the child feel ill.

A couple of weeks after the unvaccinated child inhales the droplets, she starts feeling sick.

Children often first show signs of malaise and a fever, followed later by reddish, irritated eyes, a cough and a stuffy nose as the mucus membranes and nasal passages become inflamed.

Some children at this stage develop millimeter-wide, whitish-gray bumps on the inner lining of the cheeks, as far back as the molars. For some, the spots go undetected, or do not show up at all.

Then comes the characteristic feature: the breakout of a red rash, starting on the face and spreading down the neck, trunk, and extremities.

Many of these symptoms should resolve themselves. The rash can last up to a week, often fading along the same route it appeared. The cough can last for up to two weeks after the illness has resolved. But a fever lasting beyond the third or fourth day of the rash suggests that a complication could be developing — and that is where cases can become dangerous.

Even as the rash fades, the infection can spread to the lungs and other organs.

Children are typically brought to the hospital after having the body rash for several days. Most have low oxygen levels and are laboring to breathe and need support, said Dr. Summer Davies, who sees children at Covenant Children’s Hospital in Lubbock, Texas, and has been treating measles cases there since the outbreak started in late January.

“A lot of families have kind of been surprised, like, ‘Oh, my child was fine, and then all of a sudden, they’re not,’ ” she said.

That mild disease evolves into a fever as high as 104 or 105 degrees for two, three or four days. Poor fluid intake, a sore throat and diarrhea can lead to dehydration, which over time can begin to threaten kidney function.

Young children are more at risk because of their smaller anatomy and their inability to articulate symptoms clearly, explained Dr. Lara Johnson, the chief medical officer of a group of Covenant hospitals in the area.

About one in 20 children with measles will develop pneumonia, an infection in the lungs; and severe cases can be fatal.

Dr. Davies said many children admitted to her hospital recently had cases of pneumonia caused by either the measles virus or by a second pathogen that attacked while their immune systems were weakened.

The 6-year-old girl who recently died of measles in Texas had a case of pneumonia that caused fluid to build up in her left lung, making it difficult for her to breathe, according to a video interview with her parents that was posted online. She was eventually sedated and intubated, but she became too sick to survive.

One of the hallmarks of measles is what researchers call “immune amnesia,” the temporary weakening of the immune system. Measles wipes the protection children have acquired from other infections, which leaves them susceptible to other infections for several months or even years.

About one in 1,000 children who contract measles will also develop encephalitis, or inflammation of the brain tissue, which can result in permanent damage.

For infants or children who are already immunocompromised, a condition called measles inclusion body encephalitis (or MIBE) occurs when the child cannot clear the infection. It can trigger mental changes and seizures, leading to a coma and death in most patients.

Another type, called subacute sclerosing panencephalitis (SSPE), is a degenerative condition that can occur up to a decade after a measles infection. Children often first show signs of behavior change and academic decline, followed by seizures, motor issues like poor coordination and balance, and eventually death. The mortality rate approaches 95 percent.

Erica Finkelstein-Parker, a mother in Pennsylvania who lost her 8-year-old child Emmalee to the condition, had not known that the girl had had measles before she had been adopted from India as a toddler. But she noticed one day that Emmalee was tripping and falling, slumping over to one side of her chair and struggling to lift her chin off her chest during dinner.

Doctors explained that there was no cure. Emmalee passed away about five months later.

First daughter Ivanka Trump has gone public with her practice of jiu-jitsu.

In a recent Instagram post by martial artists The Valente Brothers, Trump showed off her mastered maneuvers with trainer Gui Valente.

Trump recently shared that her daughter, Arabella, first started taking classes before the entire family joined.

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Supermodel Gisele Bündchen, who has also trained with the Valente brothers and is the mother of Joaquim Valente’s child, has publicly called out the benefits of her own jiu-jitsu practice.

“I feel stronger, more confident and empowered since I started practicing self-defense,” she said in a previous Instagram post. “I feel it’s an important skill for all, but especially for us women.”

In an on-camera interview with Fox News Digital, the three Valente brothers — Pedro, Gui and Joaquim, who are based in Miami, Florida — shared why a self-defense practice like jiu-jitsu is a great physical and mental workout for the whole family.

“We have students starting as young as 3 years old and as old as 87 continuing their training,” Joaquim Valente said. “It creates an opportunity for everyone to engage.”

For kids facing bullying, the practice helps them develop the physical confidence to protect themselves and parents wind up tagging along, according to Joaquim.

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“Saturday, we call it family day here at Valente Brothers,” he said. “We have so many families that come together, and they train with their kids.”

The art of jiu-jitsu

The martial art, which originated from samurai warriors in medieval Japan, started to become popular in the U.S. during the 20th century, when then-President Teddy Roosevelt practiced it in the White House.

For the last 30 years, the Valente brothers, whose family is originally from Brazil and trained with world-renowned Brazilian martial artist Helio Gracie, have specialized in teaching jiu-jitsu as both a self-defense tool and a path to wellness.

The brothers focus on a “7-5-3 code” philosophy, which is intended to create “spiritual, mental and physical wellness.”

Joaquim Valente shared that their instruction focuses on empowering people for various situations, like being put in a headlock or preparing for a punch.

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Gui Valente added that jiu-jitsu can also provide health benefits, noting that their predecessors were doctors — including their father, Grand Master Pedro Valente Sr., who was a plastic surgeon.

“He often talked about how what he learned on the mat helped tremendously with his career,” Gui said. “All the fundamentals of jiu-jitsu, the philosophical aspect of jiu-jitsu, can be beneficial on and off the mat.”

Pedro Valente, pictured, also trains members of the U.S. military.  (Valente Brothers)

“Self-defense is a human necessity,” Pedro Valente added. “Our style of jiu-jitsu gives students this opportunity even if they’re very busy with their lives – with business and work and family – they still can come in and learn a very powerful system of self-defense but in a very safe way.”

Jiu-jitsu is an “exercise of the mind,” the brothers described, which makes it a great mindfulness practice amid the stresses of daily life.

Maneuvers to master

The brothers repeated that their 7-5-3 philosophy – which represents the seven virtues of a warrior, five keys to health and three states of mind – is lesson No. 1 in learning self-defense.

Simple techniques — like learning how to release control if someone grabs you by the wrist and pulls, or if someone places two hands around your neck with the intention to choke — are a basic necessity, Joaquim Valente noted.

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Another essential technique of jiu-jitsu is learning how to fall without being injured.

But the best self-defense technique, according to the trainers, is avoiding confrontation altogether.

“We want to think about self-defense even before the fight happens,” Gui Valente said. “When we talk about situational awareness, we also talk about teaching students risk management.”

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Pedro Valente also discussed the importance of emotional balance and developing a “sense of poise” when approached with danger.

“When you are in a state of panic, the frontal lobe of your brain is not functioning well, you’re not going to be able to rationally decide what’s the best escape route,” he told Fox News Digital.

“The best self-defense is always avoidance. If you get into a physical fight, you’re already a step behind.”

Pedro added that having emotional balance also helps to avoid “petty arguments, bickering, that many times will lead to a fight.”

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“Anticipation is key — and the only way to anticipate is to be present,” he said, emphasizing the importance of being connected to one’s surroundings.

“Presence is something that can really enhance our mental state and, at the same time, allow us to anticipate a problem that might be happening around us.”

Self-defense in schools

The three brothers, who are all fathers to young children who train, shared their goal for jiu-jitsu to be taught more widely in schools as physical education.

The brothers have been leading this movement in the Miami area, where a few instructors have been teaching in some schools.

“We consider it to be the best form of PE,” Gui Valente said. “It really complements the academics … and what develops into physical confidence, improves children’s self-esteem.”

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Pedro Valente echoed that this education is a “powerful combination of intellectual and physical confidence.”

“Jiu-jitsu — and this is something that we work on with psychologists and therapists — is one of the key ways to address these common problems that every kid faces,” Gui Valente added.

Gui Valente holds baby Arthur, the son of Joaquim Valente and model Gisele Bundchen, in his first kimono. (Valente Brothers)

The brothers also emphasized that jiu-jitsu has helped students of all ages with weight loss due to the physical elements of the sport, as well as nutritional awareness.

“Self-defense is a human necessity.”

“You work every single muscle in your body and in different ways,” Gui Valente said. “You have to be able to develop great stamina, because when you spar, you have to last for sometimes 20 to 30 minutes, or even longer than that.”

“You have to learn how to use strength efficiently, which is truly important in pretty much any exercise you choose to practice, as well as flexibility and mobility.”