They lost seven mice in two days, but that seemed unimportant; the females were already living up to their reputations, nearly all pregnant. Doc didn't know the gestation period, but he remembered that it was short.
"Funny they all started at the same time," he commented. "Must have been shipped out separately or else been kept apart while they were switched over to Mars-normal. Something interrupted their habits, anyhow."
A few nights later they learned what it was. There was a horrible squealing that woke him out of the depths of his sleep. Chris was already at the light switch. As light came on, they turned to the mouse box.
All the animals were charging about in their limited space, their little legs driving madly and their mouths open. What they lacked in size they made up in numbers, and the din was terrific.
But it didn't last. One by one, the mice began dropping to the floor of the cage. In fifteen minutes, they were all dead!
It was obviously the plague, contracted after having their metabolism switched. Women were sterile for some time after Selznik's migraine struck, and the same must have been true of the mice. They must have contracted the plague at about the same time and reached fertility together. Somehow, the plague incubation period had been shortened to fit their life span; the disease was nothing if not adaptive.
Chris prepared a slide in dull silence. The familiar cell was there when Doc looked through the microscope. He picked up one of the little creatures and cut it open, removing one of the foetuses.
"Make a film of that," he suggested.
She worked rapidly, scraping out the almost microscopic brain, dissolving out the fatty substance, and transferring the result to a film. This time, even at full magnification, there was no sign of the filaments that were always present in diseased flesh. The results were the same for the other samples they made.
"Something about the very young animal or a secretion from the mother's organs keeps the bug from working." Doc reached for a bracky weed and accepted a light from Chris without thinking of it. "Every kid I've heard about contracted the plague between the second and third year. None are born with it, none get it earlier. I've suspected this, but now here's confirmation."