Yes it usually is caused by an infectious agent such as a bacteria or virus. So, that if Lamictal can cause meningitis is probably because it may lower your immune system and possibly allow one to get an infection. I haven't read much about the side effects of Lamictal but I don't know how (other than lowering your immune system and allowing an infection in to your system by say off balancing the normal bacteria that are there) or maybe it lowers your white blood cell count and you can't fight a bacteria or virus. That may be the case. Either way it would be a low possiblity. They have to mention all the bad side effects.
mkh9
it is an inflammation of the protective layer over the brain and spinal cord. it can cause deafness, epilepsy, hydrocephalus, cognitive deficits and it can also be fatal. some of the symptoms are a headache, stiff neck, high fever and altered mental status.
i see that you've put lamictal as your question. it has been recently found out that lamictal can have menegitis as a side effect but it is only in very rare cases.
take care