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.
Diets high in fat and cholesterol (especially from animal sources) have been found to cause colon cancer. Low-fiber diets have also been associated with increased risk, but the research isn't as clear.