Feline coronavirus is a fragile thing. A soft outer envelope wrapped around a single strand of RNA. A healthy cat's immune system can destroy it easily.

So it doesn't pick that fight. It settles into the lining of her gut, hidden among trillions of bacteria, and spreads quietly through the litter tray. In multi-cat environments up to 90% of cats carry it, and most never show more than the odd soft stool.

It looks like a truce. It's really a waiting game.

What it's waiting for is a gap. A rehoming, a surgery, an infection, anything that pulls her defences down for a few weeks. When that happens, the virus starts copying itself fast.

And this is where the danger lies, because an RNA virus is a terrible copyist. Every copy carries typos. Almost every typo produces a dud that dies on the spot. But copy yourself millions of times a day and eventually, by pure bad luck, the errors land in exactly the wrong place. That's the moment a harmless gut virus becomes the FIP virus.

The change gives it one new ability: it can now get inside macrophages. These are the security guards of her immune system, the cells whose job is to swallow invaders and destroy them. Instead of being destroyed, the virus moves in, multiplies, and rides those cells out of the gut and around her whole body.

Then it gets worse. The hijacked guards pull the fire alarm, and they never stop pulling it. Her immune system answers with everything it has. But there is no enemy standing in the open to hit. The fire lands on her own tissue.

Blood vessels turn leaky. Fluid pools in the belly or the chest. Inflammation spreads to the eyes, the brain, the kidneys. Without treatment, her organs fail one after another.

So the virus itself isn't what kills her. Her own inflammation is.

There is still no vaccine vets routinely rely on for either form of the virus, so you can't simply keep it out of her. What you can work on is the odds.