Nouns with the endings -keit and -heit are always feminine
- die Gesundheit (the health)
- die Achtsamkeit (the mindfulness)
- die Kindheit (the childhood)
The last example shows us the English counterpart – the ending -hood.
The origin of these endings is an old Germanic word that meant bright appearance. From there, it shifted toward the vague sense of “state of being”.
So Gesundheit originally was the appearance, the state of being healthy.