(CNN) Before December, no child had died in US Customs and Border Protection custody in more than a decade, according to a Department of Homeland Security official.

But on Monday, a 16-year-old Guatemalan boy became the fourth Guatemalan child to die in US custody in the past six months and the fifth Guatemalan child to die after crossing the border into the US in that same time frame.

Their deaths highlight the treacherous journeys that many Central American children experience in their attempts to enter the United States and the health issues that can arise while they are in custody.

As more details emerge surrounding their deaths, here's what we've learned about other migrant children's deaths and the policy changes they prompted.

Five deaths of children after crossing border

In December, Jakelin Caal Maquin and Felipe Gómez Alonzo died within days after they and their parents were taken into the custody of CBP.

In early May, 16-year-old Guatemalan immigrant Juan de León Gutiérrez died in US custody, days after arriving at an Office of Refugee Resettlement shelter.

Last week, a 2-year-old boy from Guatemala died at a hospital in El Paso, Texas, a month after being apprehended by US immigration authorities as he crossed the border with his mother, the Guatemalan Consulate said. The boy, who was not in US custody at the time of his death, had suffered from complications of pneumonia, a consulate official said.

The 16-year-old boy who died on Monday was apprehended and processed near Hidalgo, Texas, on May 13 before being transferred to the Weslaco Border Patrol Station Sunday, CBP said in a statement. He was found unresponsive Monday morning. The cause of death is unknown, the agency said.

Jakelin , 7, died two days after she and her father were detained in December about half a mile west of the Antelope Wells port of entry in New Mexico.

Her health deteriorated during a 90-minute bus ride from the point of entry to the Lordsburg Border Patrol station. CBP said an initial screening on Jakelin showed no evidence of health issues but attorneys for the family said they sought relief from the first Border Patrol agents they encountered because Jakelin was sick and vomiting.

She had a fever and her father told agents she was not breathing, CBP said, before she was flown to the Providence Children's Hospital in El Paso, where she died the next day.

Weeks later, Felipe, 8, also died in the hospital while being held by CBP. When Felipe and his father arrived in the United States, they were detained about 3 miles west of the Paso Del Norte port of entry in El Paso, Texas, officials said.

The boy was taken to the hospital after a border agent noticed signs of illness. Medical staff first diagnosed him with a cold and later detected a fever. The boy began vomiting later that evening and was taken back to the hospital for evaluation. He died hours later, the CBP said.

Felipe and Gutiérrez were treated, released and admitted back to a hospital within hours, according to CBP and the Department of Health and Human Services' Administration for Children and Families.

Their deaths were linked to sepsis

showed they died from infections that progressed into While it's still unclear what caused Gutiérrez's death, the autopsies of Jakelin and Felipeshowed they died from infections that progressed into sepsis.

Jakelin died from a bacterial infection known as streptococcal sepsis, according to the autopsy report from the medical examiner's office in El Paso County, Texas.

The report says streptococcus bacteria was found in the girl's lungs, adrenal gland, liver and spleen. It goes on to say the infection was "rapidly progressive," which led to "multiple organ dysfunction and death." The autopsy does not indicate how or when Jakelin caught the infection.

Felipe died of Influenza B complicated by a staph bacteria infection that led to sepsis, the Guatemalan Foreign Ministry said.

Influenza B is among the viruses that cause seasonal epidemics most winters in the United States, according to the Centers for Disease Control and Prevention.

Sepsis can be spurred by any type of infection, even a minor one, and occurs when germs enter a person's body and multiply, causing illness and organ and tissue damage, according to the Centers for Disease Control and Prevention.

There is no single sign or symptom of sepsis, but rather a combination of symptoms. Since sepsis is the result of an infection, symptoms can include infection signs -- such as diarrhea, vomiting, sore throat, etc. -- as well as other symptoms such as shivering and fever, extreme pain, pale or discolored skin, sleepiness and confusion, and shortness of breath.

More medical checks were ordered

The deaths of Jakelin and Felipe prompted congressional hearings and immigration authorities to order an increase of medical checks for children in custody.

Homeland Security and CBP officials announced a series of new procedures, including secondary medical checks on all children with a focus on children under age 10.

CBP began reviewing its policies in December, with a focus on the care and custody of children under 10, both at intake and beyond 24 hours in custody, the agency said.

The US Coast Guard Medical Corps was asked to assess CBP's medical programs and recommend improvements while the Defense Department was asked to provide additional medical professionals.