A civil trial against Petco is underway in San Diego, where a 10-year-old boy died in 2013 after playing with a rat that his grandmother bought him at the pet store chain.

Aiden Pankey died hours after being taken to the hospital with severe stomach pains. The San Diego Union Tribune reported that a doctor initially diagnosed Aiden's upset stomach, fever and chills as the flu.

The boy was told to drink plenty of fluids and rest. By the following night, his symptoms had grown so severe that he was lethargic and could not walk on his own.

He was rushed to the hospital, where he died the following morning.

NBC 7 in San Diego reported that the San Diego County medical examiner ruled his cause of death as streptobacillus moniliformis, or rat bite fever.

The Centers for Disease Control and Prevention later confirmed that Aiden’s pet rat was infected with the dangerous bacteria.

Aiden had the rat, which he named Alex, for two weeks before he died, the Union Tribune reported.

The Pankey family sued Petco, claiming that the company failed to protect consumers from potentially dangerous pets. Petco attorneys said there are dangers with any pet and that it is impossible to test all rats that it sells for the disease.

Inside Edition reported that Petco attorneys said the company sold nearly 5 million rats between 2001 and 2013, but there were only 16 complaints during that time of people becoming sick with rat bite fever. The people in those cases recovered after undergoing treatment.

Petco also said that Aiden’s grandmother signed a document about health hazards when she bought the rat.

Jurors last week were shown a pamphlet given to Aiden and his grandmother when they bought the rat that included a paragraph on rat bite fever, NBC 7 reported. The pamphlet did not indicate that the bacteria can be spread by touching an infected rodent.

According to the CDC, a person can get rat bite fever from bites or scratches from infected rodents, including rats, mice and gerbils, or by eating or drinking food or drink contaminated with the bacteria.

The disease can also be contracted simply by handling an infected rodent.

Symptoms, which typically occur three to 10 days after exposure, include fever, vomiting, headache, muscle and joint pain and a rash, the CDC said. In some instances, however, symptoms are delayed up to three weeks, when any bites or scratches have already healed.

Treatment with antibiotics, usually penicillin, is highly effective, the CDC said. Death from the bacteria is rare, but possible if the disease goes untreated.

An attorney for the Pankey family posed several questions during opening statements about Petco’s actions, questioning whether Aiden’s rat should have been tested for rat bite fever before the sale, whether Petco’s warning about the disease was sufficient and whether Petco knew of other cases of rat bite fever in animals sold at the San Diego location where Aiden’s rat was purchased.

The attorney, Bibianne Fell, said San Diego County had seen two other cases of rat bite fever, both in people who bought rats from Petco.

"(Petco) had knowledge that its customers, children it was selling its pet rats to, were contracting rat bite fever from those rats and getting very, very sick," Fell said, according to NBC 7.

Fell also told jurors that in the months leading up to Aiden’s death, the supplier of Petco’s rats had multiple animals test positive for the disease.

Attorneys for Petco and the supplier, Barney’s Pet Supply, said testing each rat sold is impossible and that no suppliers in the U.S. perform universal testing.

"There are no suppliers of rats that can guarantee that they do not carry this bacteria," Kimberly Oberrecht, an attorney for Petco, said. "It's not the customary practice. No one can do it."