When the vocal folds are abused they may swell, which causes a drop in the pitch of the voice.
Also, when we get a cold, the irritating secretions generated by the sickness, or the wear and tear on the vocal folds created by the frequent coughing spasms may cause them to become swollen.
The increased mass gives rise to a deeper fundamental tone and hence, a deeper voice. We then say that we sound like we have a "frog in our throat."
Yelling a lot can cause the same problem.
If yelling persists it can cause calluses to grow on the folds. These are called vocal nodules. Vocal Nodules diminish the naturally smooth seal between the folds. This requires more energy to produce a tone and causes even more damage to the folds.
Eventually the calluses become as hard as fingernails and are difficult to get rid of. The voice quality is typically unpleasant--low, harsh and breathy.
NOTES: Information about vocal nodules in teenagers.
NOTES: Vocal nodules in singers.
NOTES: More than you really want to know about vocal nodules.