The main causes for a lump found underneath the jaw include an indication of the presence of bacterial infections, viral infections, infections caused by bug bites, such as Lyme disease, types of noncancerous growths like lipoma, tonsillitis, a salivary gland condition and inflammation, as stated by WebMD. Most lumps underneath the jaw are caused by swollen lymph glands, and are not a cause for concern. Swollen lymph glands are common when fighting off other infections and health issues as it indicates that the immune system is at work fighting off the foreign bodies.
When the lumps become larger, firmer and harder, it is an indication that the body is fighting off a serious infection. While noncancerous tumors can be surgically removed, the lumps will generally become smaller in time as the body recovers.