So imagine a very small wasp lands on your arm and, unbeknownst to you, inserts its ovipositor beneath your skin and inserts a few tiny eggs. The eggs release chemicals that command your body to grow a huge, pustulate turnip on your arm, within which the eggs hatch into little maggots, which in turn replicate asexually for a few generations until some of them turn into adult wasps, at which point they bore out of your arm turnip and fly away.

Aren't you happy you're not an oak tree?

I believe this was growing on Quercus berberidifolia, but it was also at the Donner cabin site, so it could be some weird cultivar.