The public's concern over the possible spread of Hepatitis A has fueled a closer look into a not so pleasant question: Are homeless people using trolley cars as bathrooms?

In a letter Friday, Metropolitan Transit System CEO, Paul Jablonski, addressed allegations of commonplace defecating and urinating by homeless individuals inside Trolley Cars as false, calling the instances "extremely rare."

The letter was in response to another letter, one day earlier, from the law firm of Michael Aguirre, a former San Diego City Attorney now in private practice.

Aguirre's firm is legal counsel for the Public Transit Employees Association (PTEA) which represents trolley operators and mechanics.

The complaint came from a trolley operator, according to Aguirre's partner, Maria Severson. She talked about the issue being a habit among certain members of the downtown homeless population.

In their letter to MTS, they refer to homeless riders laying out newspapers and defecating inside the trolley cars during the early morning hours.

NBC 7 spoke with homeless men and women along the 17th Street, in a downtown neighborhood east of Petco Park. Complaints have soared about a lack of available bathrooms in this area.

Darlene Carmack, who has been homeless for the past seven years, said people often use buckets and bags behind bushes and cars, but not trolleys.

"We don't mess up the trolley 'cause we have to ride, our children, our dogs have to ride it," Carmack explained.

Jia Lawton, who is also homeless, said they often dispose of human waste by throwing it down storm drains. She also had never heard of people using trolleys as restrooms.

But homeless advocate David Ross, better known as "The Water Man," because of the water bottles he brings to the neighborhood every day for the past 20 years, said it's something he has witnessed firsthand.

"They'll go defecate anywhere they can," Ross said. He believes that includes the trolleys.

Ross said the problem was aggravated a few years ago when the city removed the portable restrooms he set up near 17th street.

Daily riders of the trolley each shared their unique experiences regarding the homeless with NBC 7.

Lawrence Holt, of Santee, has never seen anyone go to the bathroom on a trolley and said he'd make it a point to say something if he did.

Another daily rider, Greg Brahnm, now refuses to sit in the rear of the trolley because he said that is where the homeless may sleep, urinate and defecate.

"People keep pressing the button on the trolley with their hands," Brahnm said. "I use my elbow--I don't touch anything on the trolley 'cause it's so contaminated, I believe."

In the response to Aguirre and Severson, MTS laid out several steps they've taken to keep passengers and employees safe amid the Hepatitis A outbreak. That includes enhanced cleaning procedures, extra security and free vaccinations for all employees.

Severson said their letter to MTS was merely a concern and no legal action is pending.

MTS spokesperson Rob Schupp told NBC 7, "There is absolutely no truth to Mr. Aguirre's assertions. The issue raised by Mr. Aguirre is not commonplace."

If they find someone has used the trolley as a restroom on a rare occasion, they take the car out of service and it receives special cleaning, according to Schupp.