Optical Disc Decay
Compact discs have been in use since the mid-1980s, while DVDs have been around since the mid-1990s. Each of these popular optical storage media feature a similar manufacturing process, once which has been proven in recent years to result in less resilient storage of data than previously indicated.
Basically, even the most well-looked after, scratch-free optical disc can become damaged, thanks to the methods used in manufacturing. The metallic layer where the data is stored is susceptible to peeling away from the base plastic of the disc, which means that using optical discs as a long-term storage solution is not a good idea.
However, it seems that there is another threat, this time from nature itself. It has been shown that older optical discs (for instance CDs purchased in the 1980s) are susceptible to a fungus that causes the metallic layer to decay. As the layer decays, so does the data, and this can result in discs that cannot be read by a Mac CD drive.
You can easily spot a disc that has decayed (either through nature or poor manufacturing) by holding it up to the light; if light comes through any gaps in the metallic layer then your data is either lost or at risk.