 -- Calling all knitters: The Oklahoma State Department of Health (OSDH) needs 5,000 purple baby hats by October.

Health officials are teaming up with the the National Center on Shaken Baby Syndrome to prevent serious brain injury that can result from forcefully shaking an infant or young child. Their campaign's name, Click for Babies, refers to the clicking sound that knitters make with their needles.

According to the center, the No. 1 trigger for shaking a baby is frustration with crying, during what the center refers to as the PURPLE period of crying.

Each letter of the word stands for scenarios that parents may experiencing with their infant. For example, the L stands for "long lasting"; babies may cry as much a five hours each day, according to the center's website.

The period of PURPLE crying, the center said, lasts from about 2 weeks of age to 3 to 4 months. Some babies experience it more severely than others, and it is often treated as colic.

Amy Terry, an adolescent-health coordinator with the OSDH, told ABC News that Oklahoma is one of 16 states participating in the Click campaign — the fourth year for the state.

"The OSDH has requested volunteers in the past, but the media response was minimal, and last year we fell short of our cap goal," Terry said. "Campaign materials were redesigned for 2017, social media marking efforts were increased, and the public response has been overwhelmingly supportive. We have received calls from people all across the country who are willing to volunteer."

The OSDH will collect caps until Oct. 1 and then distribute them to birthing hospitals and public health units with a copy of the PURPLE program to families with babies born in November and December.