(i) Antigenic variation or presence of many serotypes as in influenza, rhinovirus and in human immunodeficiency virus (HIV).
(ii) Integration of viral DNA/cDNA in the host cell genome as in hepatitis B and retrovirus.
(iii) Large animal reservoirs as in influenza and human HIV.
(iv)Infection transmitted by cells which mayor may not express
viral antigen as in HIV.
(v) Crucial cells of the immune system are infected as in HIV.
If a pathogen has most or all of these characteristics, it is difficults to produce an effective vaccine. The HIV causing AIDS at present seems to fit this description.




