As you might think, bacterial infections are caused by bacteria, and viral infections are caused by viruses. Perhaps the most important distinction between bacteria and viruses is that antibiotic drugs usually kill bacteria, but they aren't effective against viruses.s you might think, bacterial infections are caused by bacteria, and viral infections are caused by viruses. Perhaps the most important distinction between bacteria and viruses is that antibiotic drugs usually kill bacteria, but they aren't effective against viruses.