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.
Bad bacteria in the digestive system can lead to food poisoning and illness. Food borne diseases are caused by not just the bad bacteria but also by protozoa, viruses, toxins from natural sources, and several pathogenic agents. Pathogenic bacteria are capable of harming the body by causing diseases.