The human immune system, she discovered, was not a single defense.

It was an army.

It had layers upon layers of cells that hunted, cells that remembered, cells that learned.

When a human encountered a new threat, a germ or a poison or a parasite, the immune system did not simply fight it.

It studied it.

It built weapons specifically shaped to destroy it.

And then it kept those weapons in some cases for a lifetime, so that if the threat ever came back, it would be crushed at once.

It had evolved that way, the human texts explained, because Earth was a world crowded with threats.

Bacteria, viruses, fungi, parasites, venoms, toxins.

Billions of years of life had filled every corner of the planet with things that wanted to eat, poison, or infect every other thing.

And every living creature on Earth had been shaped by that endless struggle.

Then Sepha found the pediatric literature, and she had to stop reading for a while.

The human texts on caring for young children were long and loving and extraordinarily detailed.

They were written, plainly, by people who adored their children and would do anything to protect them.

They described the vaccines that human parents gave their children to train the immune system safely against dangerous diseases.

They described the gentle care given during fevers and how parents kept watch at night.

They described with great warmth the importance of letting children play outdoors in gardens and parks and mud so that their bodies would grow strong.

And among all of this warmth, in a perfectly ordinary tone, they listed the illnesses that an ordinary, healthy, well-cared-for human child might be expected to shake off before the age of five.

Eight to 12 common colds a year, several bouts of stomach illness, ear infections, throat infections, a long list of childhood rashes and fevers, each with a cheerful name, insect stings, scraped knees, which humans regarded as practically a right of passage.

Every one of these, Sepha realized, was a battle.

Everyone was a threat that the small human body met, fought, studied, and remembered, while their parents kept them warm and fed and safe.

By the time a human child was five, its immune system had fought and won more battles than a Lirans would in an entire lifetime.

Liran children on Sepha’s own home world grew up in clean, quiet cities on a gentle planet.