The most common virus is the common cold, which is actually caused by several different families of viruses. Even though you may experience upper-respiratory symptoms associated with a bacterial infection, most cases of the common cold are not caused by bacteria-which make antibiotics useless against them.
Colon infection is caused when harmful bacteria resides in the colon. Normally, human immune system kills the harmful bacteria but there are some strains of bacteria that grow uninterruptedly even with strong immune system. When such bacteria overgrow in the large intestine, they can cause lot of complication.
Gum diseases such as gingivitis and periodontitis can cause bad breath. These diseases cause bad breath because not only are dead cells (caused by the disease) metabolized by anaerobic bacteria, but also living tissues and bone are eaten away by the bacteria, producing large amounts of VOCs.