She can't get sick?

The way it works is this, whatever biological transformation is responsible for her not showing any physical changes with aging also means she will never develop an illness, other than maybe a head cold. Developing cancer or diabetes, etc are caused by the same mechanisms that cause aging.

Anyway that is how I interpreted it.

..*.. TxMike ..*..
Take a risk, Take a chance, Make a change. Kelly Clarkson - Breakaway

When you start a movie with such a completely gibberish premise, anything is possible, however nonsensical. So don't try to make sense of it

A much cleverer premise in the film "Death Becomes Her" (Bruce Willis, Meryl Streep) suggests even if you are "immortal", stuff will happen to you -- you'll break bones or get in car wrecks or be disfigured. THEN WHAT? Being immortal doesn't mean you can't hurt yourself. Even young people can get in accidents or be disfigured.

What if you were paralyzed from the neck down in an auto wreck -- and had to live FOREVER like that? or blinded? or having had a leg amputated?

The characters in "Death Becomes Her" end up not as nubile young women who live forever, but as ghouls packed with paint and cosmetics -- like living mummies who cannot die.

I thought this film didn't even scratch the surface of earlier, more interesting films (and even TV shows) about immortality. It is not a remarkable or new theme or situation and has been explored in MANY films, TV shows, novels, etc.

She also went 100+ years only getting one small scar on her hand.