A man who showed up to the doctors with fevers and swelling discovered that his problems were caused by worms dancing around in his scrotum.

The 26-year-old man had been suffering from low-level fever, and pain and swelling of his scrotum for around a month before he was seen at an outpatient clinic at a urology clinic in New Delhi, India. According to a case report in the New England Journal of Medicine, upon examination, he was found to have tenderness and swelling in the right hand side of his scrotum.

He was taken for an ultrasound, where the problem became apparent. 

"An ultrasound examination of the scrotum showed anechoic tubular channels near the right testis and epididymal [tube that connects to the testicles] head," the team write in their explanation of an accompanying video.

 "Linear structures could be seen moving within one of the channels, a finding in lymphatic filariasis known as 'filarial dance sign'."

Or, to put it in a far worse way, the "dance sign" is caused by the "undulations of live worms" inside dilated lymphatic channels.

^{A separate video showing the beautiful dance of the scrotal worms.}

As explained in the case report, the thread-like worms are mosquito-transmitted nematodes (roundworms) of the species Wuchereria bancrofti. The parasitic worms love to hang out in the human lymphatic system and blood channels, after being deposited inside humans by mosquitos. The worms can live inside you for six to eight years if left untreated, producing millions of offspring as they do so.

Fortunately, the man's worms cleared up after a three-week course of the anti-parasitic drug diethylcarbamazine. A second ultrasound of his scrotum revealed the worms were gone, and the party was over.

The case report is published in the New England Journal of Medicine.