One father's goodbye routine with his sick daughter is going viral.

In a video that's gone viral, father Josh Trimble is seen thumping his chest in front of his daughter, six-month-old Opal Rose — who is recovering from acute flaccid myelitis, a disease similar to polio— before he says goodbye. She does the same. While it may seem like a cute little interaction between a father and hospitalized daughter, it has a deeper meaning than that.

"It’s a sign that you and daddy share a heartbeat,” he tells Fox 13.

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In an interview with Good Morning America, Trimble said that day, in particular, they were working on mobility with the baby. In fact, they focused on arm movement. So the fact that she mimicked her father in such a way was very encouraging.

According to the United States Department For Advancing Translational Sciences, acute flaccid myelitis is pretty rare. It's a disease that targets the spinal cord and parts of the nervous system.  It impacts how signals are sent to the brain, and it can cause weakness in the arms or legs and in some cases, it can impact control over one's head, neck, and even face. In more severe instances, it can cause respiratory failure.

Unfortunately, there's no treatment for this disease, but doctors do have a rundown of ways to prevent it. They recommend keeping up to date on polio vaccines and minimizing exposure to mosquitos. They also say that, while adults can develop the disease, a majority of cases are found in children. With that in mind, however, at six-months-old, Opal is the youngest case of acute flaccid myelitis documented.

According to her parents, they discovered she had the disease after they received a call from Opal's daycare. They said she was having issues raising her arms and even had trouble breathing. From there, she spent time in the hospital where she received some experimental treatments to help combat the symptoms. While she's not out of the woods yet, the fact that she's mimicking her father's chest bump is very encouraging.

Keeping a baby in the hospital is heartbreaking for any parent, but small moments like the one Opal shared with her father are great for keeping morale up during a trying part of life. Thankfully, it's looking like the young fighter is doing well. Those interested can keep track of her journey on the Facebook page, Opal's Fight — Team Trimble.

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