We are living in a sea of germs. They are in the air we breathe, the food we eat, and the water we drink and swim in. Many of the viruses are no longer bothering humans because humans have developed more advanced immune system to counter the teeming viruses that exist in the world. However, some brute viruses such as Variola Virus, causes smallpox, once was a threatening virus to the world. The virus was spread through the air and the smallpox infection resulted people in high fever, serve aches, scarring rashes or even death. Thus vaccines were introduced to prevent contagious disease, with the help of knowledge of human immune system. The human immune system is complex and it holds three different defense lines to prevent pathogens. When