An apparently healthy mother and her teenage son have died from a mysterious illness just four days apart, leaving doctors completely baffled.

Zack Smith, 19, passed away at a hospital in St Peters, Mo., on Sunday and his mother, 48-year-old Rhonda Smith, died on Thursday morning, KMOV reported. Although physicians have suggested the victims to have been suffering from a particularly virulent strain of the flu, the St. Charles County Health Department is still working with infectious disease doctors to figure out what led to the pair's sudden decline in health.

After experiencing flu-like symptoms and coughing up blood on Friday, the 19-year-old was diagnosed with pneumonia and a staph infection by doctors at St. Joseph's health Center. However shortly after, his organs began shutting down and he died from cardiac arrest on Sunday morning, according to a GoFundMe page set up for the family.

Zack's family was told that an earlier flu shot had failed to eradicate a strand of the flu, resulting in his death. "They said the explanation of death was even more to the fact they felt it was a strand of the flu from Africa," relative Scott Dyer told KMOV.

"So completely destroyed inside and out," Rhonda posted on her Facebook page on Sunday. "My sweet precious son Zack lost the fight of his life this morning. I am still at a loss of understanding how I walked in to the ER with him just 2 short days ago and now he is gone...I just want you back baby boy. I love you so so so much."

But on Wednesday, Rhonda had to be rushed to Barnes St. Peters Hospital after she felt hot and complained of a pain in her side, according to UK MailOnline.

"While we all thought she was just heartbroken and depressed from losing her son we took her to the urgent care," the gofundme page explains. "They immediately referred us to the ER due to her blood sugar levels greatly above what their meter could read."

Although she did not have diabetes, doctors said she was at risk of slipping into a diabetic coma because her blood sugar levels were so high. After experiencing difficulty in breathing, her heart stopped eight times but was repeatedly revived until the doctors couldn't find a pulse for the eighth time.

Less than 12 hours after being admitted, she died on Thursday.

"My sister went to heaven early this morning and is now resting peacefully with her son and mother," Rhonda's sister Wanda wrote on Facebook on Thursday. "I am extremely saddened by this. Having a very hard time coping with losing 2 close members of my family in less than 72hrs."

Rhonda's family members have been advised to get tested for the flu and chest X-rays just to be on the safe side.

"Any prayers sent our way would be more than appreciated," relatives wrote on the gofundme page. "We are all very frightened and hurting in this tragic time."

Meanwhile, an autopsy is being carried out to find the cause of Rhonda's death, with the health department hoping to know more once lab tests are completed.

Rhonda leaves behind three more children, aged 12, 16 and 21.