You can pretty much cure anything at all. The common cold is somewhat elusive, but pretty much all fleshy fish can be cured. Gravadlax, or gravlax, is what I’d call a soft and loose cure. The salmon remains moist and almost raw in the middle and the curing process can take anything from 24-72 hours.
The salt and sugar mixture is usually just flavoured with dill, although beetroot is becoming a popular addition. Adding toasted caraway seeds gives it a hint of Nordicness (which is also becoming more popular) but crushed toasted coriander seeds give a more pleasant citrusy result.
You can add grated ginger, chopped lemongrass, chilli or lime leaves, and various spices from star anise to vanilla to good effect. The chemical reaction that happens when you cure a protein in salt or sugar (or more commonly, both) is that the moisture is literally sucked out of the protein by the salt and sugar crystals.
They love nothing more than moisture (that’s why a meringue will go soggy if not kept in an airtight container, and why those annoying salt shakers block up) and so they serve a good purpose in food preparation. They also kill surface bacteria which is why salted meats, mutton birds and the curing of small goods always involve salt.