Don’t open your mouth.

That’s advice usually given to criminal defendants, protesters in totalitarian countries and headstrong politicians. Nowadays, it’s what health experts say must be done by Olympic sailors, rowers and open-water swimmers competing in the human waste-laden waters of Rio de Janeiro.

On that disquieting note, the opening ceremony takes place tonight.

The contaminated water that officials promised to treat but never did is a metaphor for a disturbing run-up to these Olympic Games — staged by Brazil, a country in political and economic crisis, and by a city struggling with street crime and poor infrastructure.

At their best, the games celebrate athletic achievement and our common humanity amid a marvelous panoply of cultural diversity. But as we see time and again, the Olympics are not an escape from the world but a reflection of it.

Friday night’s opening ceremony, featuring 10,000 athletes marching into Maracanã Stadium, will be a glorious display of pride, patriotism and swirling colors. But you also will notice:

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n The first Muslim American woman to compete in a hijab. Ibtihaj Muhammad is a fencer who wears the head scarf under her mask. She’s from New Jersey, trains in Manhattan and could win a medal. She also is a potent reminder of the contributions Muslims make to this country.

n The first-ever refugee Olympic team. The 10 athletes come from Syria, South Sudan, Ethiopia and Congo, countries contributing to the worst refugee crisis since World War II. Yusra Mardini, an 18-year-old Syrian swimmer, jumped into the Mediterranean Sea when her boat broke down short of Greece and, with her sister, swam more than three hours alongside to guide it ashore safely. These athletes are true portraits in courage and the definition of inspirational.

n Missing Russians. Nearly 120 were banned because of their country’s massive state-run doping program. Drugs have been a scourge on sports. But Russian President Vladimir Putin blamed the ban on politics intruding on sport, a reminder of current geopolitical tensions.

n The United States contingent, including 30 from New York. Flag-bearer Michael Phelps, the swimmer and most-decorated Olympian of all time, is coming back from retirement — and from a second guilty plea to drunken driving, in 2014. Our sports icons are mortals, too.

Security. It might not be noticeable in the stadium, but will be impossible to miss on the streets — more than 100,000 security personnel, an Olympic record. These are uncertain times.

We hope Rio pulls it off. We also hope the city derives some lasting benefits in housing, infrastructure and the like. But the modern Olympic legacy of facilities that sit idle afterward makes the games’ cost — Brazil is spending about $11 billion — indefensible. Perhaps the time has come to consider a permanent location.

Once the games begin, the action often takes over the story. We hope the competition is fierce but fair, everyone comes home safe and no one gets sick from that water. Then we hope the Olympic movement reflects in earnest on how to make itself better. — The editorial board