Infectious diseases are very important in terms of epidemic and bioterrorism. Some infectious diseases like tuberculosis re-emerging as epidemic throughout the world. Sexually transmitted diseases (STD) especially HIV and AIDS, although it is declining in developed countries due to early diagnosis and use of anti-retroviral drugs, but is still increasing in number in under developed and developing countries because late diagnosis and costly treatment. Infectious diseases like malaria and leprosy are endemic in some African and South-East Asian countries but can pose a threat to developed countries too. Sudden outbreak of influenza, SARS and NIPAH virus simultaneously in different countries can lead to havoc. Therefore, to prevent morbidity and mortality in neonates, children, adults due to various infectious diseases, it is essential to follow the vaccination schedule properly. List of agents of bioterrorism- category A, B and C should be known to everybody for own safety. All these have been highlighted in this chapter.