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A photo of a quarantined bus thought to be infected with whooping cough popped up on Twitter on Tuesday, but there was no official word from TriMet.

(Photo by Twitter user Daily Cyclist)

TriMet quarantined a No. 10 bus Tuesday after its driver was diagnosed with Whooping Cough, a highly contagious respiratory infection that is especially deadly in young children and pregnant women.

However, Multnomah County's chief health officer said riders on the bus had a "low likelihood" of being infected. In fact, he said the case remains unconfirmed.

"We don't want to create a panic," said Justin Denny, Multnomah County health officer. "You'd have to receive significant exposure at close proximity, possibly for several hours, for it to be a serious concern."

TriMet knew on Tuesday that one of its bus drivers had been diagnosed with whooping cough, or Pertussis, but news of the infection first popped up on Twitter.

At 3:57 p.m. Tuesday, a TriMet employee going by "Daily Cyclist" tweeted a photo of a bus with "caution" tape parked away from other vehicles at Southeast Portland's Center Street garage.

Plastic yellow tape crisscrossed the front door of bus 2609. A hand-written sign read, "Do not enter. Infected bus."

The Oregonian contacted Oregon's largest public transit agency about the photo on Tuesday afternoon. A spokeswoman said she didn't have information about the case. About noon Wednesday, more than 24 hours after the bus was pulled out of service, TriMet eventually released a statement that said an operator behind the wheel of the No. 10 was diagnosed with the contagion on Wednesday night.

"The operator was taken to the doctor after the run, was diagnosed and received treatment," said TriMet spokeswoman Angela Murphy.

Murphy says the operator drove the Line 10 route during the rush hours between 5:30 a.m. to 9 a.m. on Tuesday. If riders have concerns about infection, she said, they should consult a physician, the local health department or call the Oregon Public Health Division Hotline at 971-673-1222.

Public bus drivers are not required to receive vaccinations under Oregon regulations to receive a commercial driver license, and TriMet does not have the authority to require vaccinations. When it comes to working sick, the agency simply asks employees to stay home if they are feeling ill.

Meanwhile, Denny said no one had officially notified county health officials about the case of whooping cough.

Denny said the general public tends overreact to the threat of whooping cough. In fact, more people die every year from the flu than Pertussis, he said.

"A couple things are working in our favor," Denny said. "For one thing, the driver was facing forward. Also, most people are vaccinated."

&amp;lt;a href="http://polldaddy.com/poll/7540686/"&amp;gt;Does TriMet have a responsibility to notify the public when they believe a vehicle is infected with a highly contagious disease?&amp;lt;/a&

Still, Bruce Hansen, president of

, said the incident raised serious questions about TriMet's obligation to notify the public in timely fashion when it suspects the public or operators have been exposed to a serious illness.

Hansen said he wasn't notified until he received a call from a member of the maintenance crew charged with disinfecting the vehicle on Tuesday night.

"They were given respirators and told to spray it down with a cleaner," he said. "Apparently, the vehicle has been deemed clean enough" for service.

Hansen said he had been told that the operator of the bus was checked over by the company's physician before being allowed to go home. "I don't have much more information than that," he said. "No one from TriMet has talked to me."

The union president criticized the agency for not being more open about what he called "disease scares" on its vehicles.

About five years ago, he said, a bus driver was diagnosed with whooping cough. "They haven't been forthcoming with information about what really is an ongoing health concern among operators," Hansen said. "We're always in contact with riders, touching hands to give out transfers, so it's a job hazard."

TriMet officials say they take public health concerns seriously. Roberta Altstadt, an agency spokeswoman, said TriMet didn't want to release information about the driver diagnosed with whooping cough until after it could consult federal medical privacy laws and talk to county health officials.

Also, maintenance workers were given dust masks, not respirators, as Hansen claims, Altstadt said. "Once the involved operator left the bus, there was no longer an infectious concern," she said. "Merely airing out the bus would have eliminated any residual pertussis bacteria."

Saying the bus was quarantined, Altstadt said, wasn't accurate. The terms "blocked off," "set aside" or "isolated" would be more accurate, she said. "It was cordoned off, albeit a bit overzealously, as a precaution."

Riding with germs and possible diseases is part of commuting by public transit.

Two years ago, after TriMet made drastic cuts to its bus-cleaning operation, The Oregonian commissioned a team of Portland State University microbiologists to test seats, hand straps and rails on various vehicles. The scientists found oxacillin-resistant staphylococcus aureus among the 120 bacteria colonies found on trains and buses. Commonly known as MRSA, the bug is notorious for rejecting antibiotics, eating flesh and causing pneumonia.

Whooping cough, on the other hand, is a bacterial infection that can spread through the air via microscopic droplets when people cough. People with whooping cough have coughing or sneezing spells in which they can’t catch their breath between coughs, causing them to make "whoop" sound as they attempt to catch their breath.

In the 1970s, health authorities believed they had all but vanquished the sickness, but cases have popped up increasingly in recent years.

Nationally, 48,277 cases of whooping cough were reported in 2012, which is the highest number reported in the U.S. since 1955.

Experts blame the spike on fewer parents vaccinating their children as well as a switch from "whole cell" pertussis vaccine to a more purified "acellular" mix, which wear off in children by about the age 10 and requires a booster shot.

"People need to remember to get that booster," said Susan Wickstrom, a spokeswoman for the Oregon Health Authority's public health division.

In 2012, Oregon had more than 900 whooping cough cases, the most since the 1950s, Wickstrom said. In the past decade, four infants have died from Pertussis in Oregon. "It affects babies the worst," she said.

But Denny said the number of whooping cough case in Multnomah County have dropped significantly in 2013, with 71 reported so far. "Of course," he said, "this wouldn't be an issue if people just didn't come to work when they're sick."

-- Joseph Rose