Mundanes, also called Mundies, are the normal human kind. They were created by god and get protected by the nephilim.
Most mundanes are oblivious of the existence of the Downworld, but a few have the Sight, that makes them able to see through the glamour of the downworlders and nephilim (that they put on to hide from them).

Even if mundanes are born without the sight or connections to the downworld, they can still be dragged into it, through several ways. Choose your favourite.

1. Being bitten by a werewolf and turning into one

2. Being fed vampire blood and bitten + a little death on top and then you're a vampire

3. Being kidnapped by a faerie and being turned into a 'changeling'

4. Learning magic and then voilà: You're consired a witch

5. Drinking from the Mortal Cup and there's a little potential that you'll turn into a nephilim

Mundanes can sometimes have children with demons. Sometimes, demons may (or may not need to) disguise themselves as humans and seduce mundanes, resulting in a warlock child.

If a mundane gets a child with a Shadowhunter, the child is most likely going to be a nephilim, simply because Angelic Blood is dominant.

Article by MountainChild01