Why it is difficult to develop vaccines for some diseases ?

It is difficult to develop vaccines against the diseases caused by viruses. Viruses are very specific to hosts. They live and multiply only in the living cells. They cannot be cultured on artificial medium. It is because of these factors that vaccines are difficult to be prepared in such cases. However, living cells of human's body which are exposed to action of a virus, secrete a heat stable basic antiviral, all visitors to a foreign country are vaccinated against the disease which is not preventable in that country.