There is sometimes a fungus that can develop on the lens of a very old camera or one that has not been cleaned very often. This is seen as a dull substance on the glass that is not removed with a glass cleaner. Use a light coating of mineral oil and let stand overnight, then wash with Windex or other glass cleaner until all residue is gone.
Dust in a camera can be a real problem. Canned air, available at most computer stores for cleaning keyboards and computers is a low cost option to dust out the camera on a regular basis. If you do this routinely and keep the camera in a case when not in use dust will not become a problem.