Bacteria cause bacterial infections and viruses cause viral infections. Antibiotic drugs usually kill bacteria, but they are not effective against viruses. Infections caused by bacteria include strep throat, tuberculosis and urinary tract infections. Diseases caused by viruses include chickenpox, AIDS and common colds. In some cases, it may be difficult to determine whether a bacterium or a virus is causing your symptoms. Many ailments such as pneumonia, meningitis and diarrhea can be caused by either type of microbe.