The best defense against viral or bacterial disease is all the above. Intact skin, the epidermis is the first line of defense. It creates a barrier against infection. You also have normal flora on your skin that fights pathogens.
Good hygiene is also important in the fight against disease. We are bombarded by pathogens daily, good hygiene will keep them at bay.
A healthy, functioning immune system is also paramount in the fight against disease. Most of us don't get ill because of our immune systems. Compare this to someone who is immunocompromised, they get ill frequently and stay sick longer. If immunocompromise is severe, they can not fight off pathogenic organisms and perish from opportunistic infections.
Your own immune system is the best defense against viruses. Because viruses use the body cells' own DNA to replicate, it is difficult to formulate drugs to attack them--those drugs would then affect the rest of the cells, as well. But your own immune system usually can deal with the cells. As charles85 points out, antibiotics are useless against viruses--they break down cell membranes, which viruses do not have.
Your skin is another effective barrier. Even HIV usually enters through breaks in the skin.
Hygiene also plays a big role, which is why people are always telling you to wash your hands! It is one of the best ways to avoid getting sick.
Vaccination is the best mehtod to avoid viral diseases! Till now, we cant even succeed common cold u know by antibiotics..If you go to doctor, cold will leave you in 7 days, if you don't it will leave you in a week ;-)
Hygiene and Immunology both are the best ways!!!
skin can't stop all the viruses..Of course HIV is a dreadful virus.