Infinity is both a number and a concept. I asked my 10 year old niece what kind of number infinity might be and she said, "It's a composite number." But I want to think about weather infinity is a prime number? Clearly if you divide infinity by any number, you get infinity. Also if you divide any number by infinity you get zero. Not sure this helps any. If infinity divided by any number is still infinity, then perhaps infinity is a prime number after all. Though certainly not a normal kind of prime number.