I wouldn't say that vegetarianism causes disease, but sometimes it does result in certain nutritional deficiencies that we typically get from animal sources. The long term of those nutritional deficiencies could result in some problems.
I would say though, that a moderately educated vegetarian would be able to establish nutritionally balanced meals through identifying these deficiencies.
make sure you get vitamin b12 daily - which can only be found in meat (now cereals (some) fortify it in there or a supplement). Also, make sure you are getting complete proteins everyday. Since it's only found in 1 item of meat, you need 2 items of the following:
peanut butter sandwich (bread)
hummus (chickpea spread) and bread/pita
I believe Quinoa is the only grain that is actually complete by itself... please do some research on complete proteins if you don't eat meat.
I think egg might be a complete protein..look it up to be sure... otherwise you should be fine...