After being sick for the past six weeks I'm pretty sure it is the chard and beets that I have been eating recently.  I have had food troubles for a few years (gluten, eggs, and chilies) and identified beets about a year ago as one of the things that made me ill.  Hoping it had gone away, I started to eat them again recently with horrid results.  I have had fatigue, muscle weakness, digestive troubles (gas and bloating), abdomen pain, a bitter taste in my mouth, and other not-so-fun symptoms.  I've been post chart/beet for a few days now, and my symptoms are clearing up.  

But to answer your question, I have read quite a bit on food allergies and have heard of allergies to just about everything (not necessarily to beets specifically).  My understanding is that for food allergies, your body, for some reason, reacts to a particular protein in a particular food (each food or food family has proteins specific to that food) as if it were a virus.  I don't think anyone has identified why one person's body reacts to certain food proteins as if they were harmful viruses. 

It certainly is a strange ride, though, huh?