A mysterious respiratory illness that infects dogs is spreading across the US. Here's how to protect your pooch, and what to do if you think they have it.

• Dogs across the country have been falling ill with an unidentified sickness.
• Its symptoms include coughing, sneezing, runny nose, and energy loss.
• If symptoms persist, the disease can develop into pneumonia, which has been fatal in a few cases.

A mysterious respiratory illness is spreading between dogs across the US, and veterinarians aren't sure what's causing it. In some cases, it's been deadly.

It's not uncommon for dogs to develop a cough this time of year, Dr. Sorin McKnight, an associate veterinarian at Wellborn Road Veterinary Medical Center in Texas, told Business Insider.