But what if we could change that by growing organs?
It isn't the premise of a science-fiction movie. Scientists believe they have the knowledge and technology to make this scenario a reality.
Researchers are in the very early stages of using adult stem cells to grow human organs. The twist: These human organs are being grown inside animals.
Every day, about 22 people in the United States die while waiting for organ transplants, according to federal statistics.
In an attempt to solve the global donor-organ shortage, researchers at the University of California, Davis have created embryos that have both human and pig cells.
These cells are created by taking human stem cells from an adult's skin or hair, using them in a pig embryoand injecting it into the uterus of a pig.
The embryo needs a few weeks to mature for scientists to determine whether the procedure worked, but after 28 days, the pigs' pregnancies were terminated, and the cell remnants were analyzed.
Besides growing organs for transplant patients, this technology may help treat people with life-threatening diseases like diabetes, said scientist Juan Carlos Izpisua Belmonte of the Salk Institute for Biological Studies in La Jolla, California.
Belmonte is working with UC-Davis' Pablo Ross on this research. Their work is being funded in part by the Defense Department and the California Institute for Regenerative Medicine.
Getting to the point of creating human-animal hybrid organs is possible because of the combination of two breakthrough techniques in stem cell biology and gene-editing technology.
Although the risk of an animal acquiring human consciousness is slim -- their brains are smaller than and different from humans' -- anything is possible, Belmonte said.
"We need to consider all the possibilities. Where the cells go is a major question. They can go to the brain or anywhere," he said.
But the National Institutes of Health's funding ban has drawn criticism from scientists, including Daniel Garry, a cardiologist who leads a chimera project at the University of Minnesota, who said the agency's stance is inhibiting medical progress and creating a stigma around the research.
'Personalizing' the future of medicine
Other than transforming to essentially anything, human stem cells are important in chimera research because they can limit the chance of a human-pig organ being rejected by a transplant patient's body.
"With the compatibility of these cells, this will open the door for personalizing medicine," Belmonte said.
So why grow organs inside a pig, anyway?
The creature's organs are almost the same size as ours, said Walter Low, professor at the University of Minnesota's Department of Neurosurgery.
"The animal is acting as a biological incubator," Low said. "If the stem cells were taken from a patient with diabetes, those stem cells are identical to the patient's own cells."
Low has been using stem cells to treat neurological disorders such as Parkinson's disease, a condition that affects the central nervous system.
This idea of chimera organs isn't a new concept, Low explained. In 2010, Japanese scientist Hiromitsu Nakauchi, who is now a stem cell biologist at Stanford University, was able to grow a rat pancreas inside a mouse. This was a huge breakthrough in chimera research because rats and mice are different species.
If we can grow a rat organ in a mouse, why not a human organ inside a pig? Low asked.
However, we're not very close to growing a human organ inside a pig just yet.
Belmonte and other scientists are pushing forward in their research. They want to create life-saving organs and potential treatments for debilitating diseases, but that dream isn't possible without funding and support, Belmonte said.
"If this works, it may change the practice of medicine," he said.