Desperate to discover a cure for the cyclical 48-year-fever, known as Trailmen’s fever, Dr. Randall Forth persuades a colleague, Dr. Jay Allison, to undergo hypnosis. He calls forth a secondary personality, Jason Allison, who is gregarious and an experienced mountain climber, while Dr. Jay Allison is a cold, clinical man with no outdoor skills.
Jason is asked to lead an expedition into the Hellers to collect medical volunteers from among the Trailmen. Accompanying him are Rafe Scott, Regis Hastur, Kyla Raineach, a Renunciate guide, and several others. During the trip, Jay/Jason yo-yos between his two personalities – one warm and charming, the other distant and clinical. Jason, the warm personality, falls in love with Kyla.
They are attacked on the trail by a party of hostile Trailwomen. As a result of the attack, the Jay personality reappears, and is considerably more formal than the Jason personality. When they reach the Trailmen nest where Jay/Jason lived as a child, he is recognized. The party is invited into the Trailmen’s tree habitat.
The Old Ones of the Sky People (Trailmen) inquire why Jay/Jason has brought an armed party of humans to their nest. Jay/Jason explains his mission, to find a remedy for 48-year-fever. He introduces Regis Hastur to the Old Ones, and Regis also pleads for the Sky People’s assistance. One hundred Trailmen volunteer. The party, with volunteers, returns to the Terran Trade City.
Some months later, a serum is developed for the treatment of 48-year-fever. Regis Hastur arrives to congratulate Jay/Jason Allison. The exposure to Regis reminds Jay/Jason of the expedition, and causes Jay/Jason to merge into a third, more stable personality.

Question: How often does Trailmen's fever occur?
Answer: Every 48 years. 
Alternative answer: Every 48 years.