“A month later over half the colony’s 2,000 person complement was dead, and more were dying. The planet was quarantined. Earth shipped supply capsules, charging their cost against the colony’s Earth-side performance bond, but refused to accept any person or anything from the settlement itself. Thirty-six days after the onset—officially fixed at the moment of the first victim’s initial shakes—ten additional men and women suffered the warning siege and set their affairs in order, each in the manner befitting himself and his religion. On the following day no new cases were reported; nor were any on the days thereafter. The ten recovered, and the epidemic (for so it was then regarded) was over, as mysteriously as it had commenced. The colony was held in quarantine for five years, during which time it accumulated a debt it would take a century to exonerate, but there was no recurrence, either there or anywhere else.

“Fifteen years later the chill broke out again, however, at a colony twenty-five light-years distant from the first. The pattern was identical, with the exception that the authorities alertly slapped on the quarantine within hours of the first death. Half the pioneers had been fatally infected within thirty-six days; the rest lived. Humanity breathed a collective sigh of relief when no contagion was detected.

“Now the debate of the first century § raged hotly over the chill. What was it? How did it spread? For the first question there was no satisfactory answer. For the second there were several. One vociferous group held that the chill propagated by etheric waves traveling at the speed of light, a kind of death ray engulfing entire planets and moving on after a suitable interval to others. This was quickly labeled the Wave theory. Another leading group claimed that the contamination spread by personal contact, transmitted by some short-lived virus that rapidly mutated into impotence: specifically in thirty-six days. This was known as the Particle theory.