As a short answer? Yes.
The main issue with a leak anywhere in the home, beyond simple and not-so-simple water damage, is that a leak will get water anywhere in your home where it’s not supposed to be. Places which, incidentally, may contain mold spores already present, just ready for a bit of moisture to fully germinate.
Mold spores are nearly everywhere, (the amount and type depending on your locale) and the seasons of allergies are only marking the times in which they are more prevalent in the atmosphere than is usual. These microscopic problems in waiting aren’t a major concern for most people until moisture is introduced.
So the main question when you have a roof leak isn’t if it will cause a mold outbreak, but rather when and how severe.
You run the risk of developing two main types of outbreak: Systemic, in which the attic is poorly ventilated and allows for mold to develop all over the place—this will require professional aid; and limited growth where the mold only takes hold around the immediate leak—this is a bit easier to deal with, and can be handled usually by the right product.
There are a few different types of mold you can expect, again, depending on your specific location. The specific types are a bit beyond the scope of this article, but in homes, the molds break into 3 specific types.
The Allergenic, those types of mold that aggravate allergies, and is especially noticeable in those already suffering from asthma.
The Pathogenic, which primarily target and acerbate the symptoms of those already under the effects of an illness or disease.
Finally, The Toxigenic, which are the molds that specialize in toxic compounds and produce poisons; the most famous of which is Stachybotrys, more well known as the dreaded “black mold.” This type of mold has, and will continue to kill people, so please take care to examine the point of leakage.
All in all, if you spot or otherwise detect a roof leak, deal with it as soon as possible. The costs associated with a proper mold outbreak will far outweigh the immediate cost of patching a hole in the roof—even if you must seek out professional help.