What is the difference between Viral and Bacterial Infections?

In my pediatric practice like in many other medical centers, the most common problem among children visited is the infectious disease which are mostly viral in origin and to lesser extend bacterial . Of course there are many other causes or living pathogens that can make a child sick like fungus or mycoplasma, etc.. which are much less common.
Both bacteria and viruses are very small living pathogen which cannot be seen by naked eyes and you need a microscope to be able to see them. Bacteria can reproduce on their own and can survive in any environment including extreme heat and cold, most of them are not only harmless but even helpful for the body and actually lives in a harmony in the host and only 10% of them can cause disease in human , some time minor and occasionally life threatening infection.
Viruses are the most common cause of infection in children and are too many in types, maybe over 200, sitting around waiting for opportunity to enter the body , multiply and making the child sick. Viruses are tinier than bacteria in size and unlike bacteria, cannot survive without a host and can only reproduce by attaching themselves to the body’s cells. Generally a viral disease or illness is usually mild and the symptoms which they present can be multiple in types and not related to one organ this is unlike bacterial illness, like the common cold which is caused by a virus and the child may have evidence of tonsillitis, conjunctivitis and even diarrhea at the same time but bacterial infection usually involve only one system or one organ like tonsillitis of streptococcal origin which is the usual bacterial infection of tonsils or meningitis which is infection of central nervous system. So when a child with infection present with multiple unrelated symptoms it is more likely to be of viral origin. However there are some viral illnesses that make the child severely sick and differentiation with the bacterial cause can be difficult, or the symptoms of a bacterial infection some times are similar to those caused by a viral infection. This is one reason why occasionally it is difficult for the physician to decide about the causative agent and use antibiotics when it is not indicated . Also presence or absence of fever or even if temperature is really high it does not help to differentiate a viral from bacterial infection for sure but the medical history and physical examination of the patient helps a lot to differentiate the two and sometime some laboratory tests will help to make this differentiation easier.
Antibiotic can kill bacteria but is not effective against viruses like common cold or influenza viruses, beside using antibiotics without being almost sure of the diagnosis can harm the child .Over use of antibiotics also create resistant strains of bacteria in the body and the community, can kill the good bacteria in the body and help to over grow the harmful bacteria. For prevention of many viral infection vaccination stand the first such as polio , measles and chicken pox or flu, hepatitis A and B and Human Papiloma Virus which is the only vaccine available to prevent some cancers. Treatment of viral infections has also been improved and for some viral infection like herpes, HIV and Influenza antiviral medication are now available. However many times the only way to treat a viral infection is to wait and let the illness to run its course.