For patrons, the opera is a place to go to escape the rest the world, to be treated to bigger-than-life, elaborate sets and singers whose sumptuous voices reflect life's passions and pain. But behind the curtain, an opera production is a complex and delicate proposition, with numerous moving parts held together by performers, staff, technical support and directors.

Just how tenuous it all is has been brought home in recent weeks, first at the Metropolitan Opera in New York and, in the past week, at the Pittsburgh Opera and its production of Verdi's grand opera "Aida.''

It took untold hours of conception and preparation just to marshal the set and forces together for the Pittsburgh Opera's current run of "Aida." Its Triumphal Scene is one of the most complex in all of opera, with a huge chorus, solo singers, numerous extras, a horse and even a local celebrity, former professional wrestler Bruno Sammartino. Just commanding all those variables while making the whole appear polished was an achievement in itself.

Then came the cancellations.

First the scheduled Amneris, mezzo-soprano Stephanie Blythe, caught the vicious bug going around, leading to two substitutes: Marianne Cornetti on Saturday and Jane Dutton on Tuesday. And in Tuesday's performance, the illness tapped Radames, tenor Vladimir Kuzmenko. He had to withdraw after the third of the four acts, leading to the unheard-of solution of conductor Antony Walker singing from the pit while Mr. Kuzmenko acted.

At the Metropolitan Opera's recently concluded production of "Tristan and Isolde," no less than four tenors sang Tristan, with two sopranos singing Isolde due to various ailments, including one who retired mid-act. Luckily for the Met, it is the one American opera company with a system of cover singers on standby because of the number of productions it stages. A mid-sized company such as Pittsburgh Opera simply cannot pay top singers to sit around in case they are needed.

"We can't afford the luxury of setting real money aside," said Christopher Hahn, the local company's artistic director. "You can go years without it happening or you can have two cancellations in a row, so it doesn't make sense to limit what you are doing just because a crisis might happen."

It doesn't help that opera houses generally frown on the idea of another poaching its singers. So, rather than make deals ahead of time with potential replacements, the contingency plan is usually to scope out singers who could sing the roles ahead of time and keep track of them.

"You know who they are and where they are," said Mr. Hahn. "When you get into an emergency situation, the rules change." At that point, companies will often agree to let a singer under contract skip out for a short time to bail out another opera house.

At a steeper price, of course. While the Pittsburgh Opera has insurance to pay patrons in the case of a concert cancellation, it does not carry insurance to cover singers who withdraw, sticking it with the typically substantial expense of finding a replacement. Ms. Cornetti's last-minute, business-class flight from Amsterdam ran the company nearly $6,000 -- and that on top of the enhanced fee they paid her to perform at the last minute. The only good news is that the Opera does not have to pay the singers who cancel.

"They don't get paid unless they go on stage and perform," said Ellen Sheppard, director of external relations at the Opera.

The other issue for the industry is that there is not an abundance of performers who can sing the heaviest roles. At the Met, for example, even with its backups and industry sway, it still had issues landing a Tristan and Isolde because few singers exist who can master these difficult Wagner roles. Like Verdi's "Aida," it calls for a special breed of singer.

Radames, Amneris and Aida demand huge voices -- ones whose vocal cords have matured and strengthened to project over the large orchestra.

"There has been this proliferation of opera companies in the states in the last 30 to 40 years," said Mr. Hahn. "But the bigger voices are a natural phenomenon. Only a percentage of voices develop that way. They are not hanging on trees."

"If you were looking at a light lyric role like Mimi [from Puccini's "La Boheme"], you should be able to find many people that can fill in for it," said F. Paul Driscoll, editor-in-chief of Opera News. "But there are certain requirements involved in doing these big roles."

That's why the Pittsburgh Opera Center artists, and other younger singers at similar training programs around the country, cannot cover these roles.

"They would not be ready vocally to sing Radames," said Mr. Driscoll. "The company has an obligation to its young singers to show them off in the roles that will show them off the best." And trying such singing could do permanent damage to their cords and careers, said Mr. Hahn.

At the end of the day, the number of singers who can sing Tristan or Radames at a high level is small indeed. Tristans are generally thought to be a near endangered species numbering at about 10 worldwide, and true dramatic tenors for big Verdi roles don't number much greater.

"It sounds crazy, but ... there are not that many Radames out there," Mr. Hahn said.

He ultimately has decided to cover both principal roles for the rest of the "Aida" run at the Benedum Center, Downtown. He has hired tenor Stephen O'Mara to sing for Mr. Kuzmenko on Friday and to cover for him on Sunday. And has retained Ms. Dutton to stand by in the event that Ms. Blythe also cannot sing either of the remaining performances.

What's interesting is that all of this off-stage drama engages and even attracts audiences as much as the theatrical sort.

"It is live drama, and people are suddenly reminded of the difficulties of it and it becomes a spectator sport. Humans like that," Mr. Hahn said. "You want to see the athlete teetering on the edge and you want to see how disaster is averted."

As for the heroics of Mr. Walker singing and conducting from the pit? Neither Mr. Hahn or Mr. Driscoll has ever heard of that happening before.

"I have heard several times singers singing from the side of the stage, but not conductors," said Mr. Driscoll. In fact, the former happened at the Benedum in 2005 when soprano Jane Eaglen sang while Susan Marie Pierson acted on stage in the Pittsburgh Opera's production of "Fidelio."

"Under 5 percent of conductors have professional voices good enough," said Mr. Driscoll. "I am just praying that our Aida stays well," said Mr. Hahn, with a laugh. "She has not gotten sick yet. Needless to say, I am checking up on the where other Aidas are."