We have vaccines for nearly thirty of the more than seventy infectious diseases which are pathogenic for humans. Most of the vaccines, especially those to prevent childhood diseases, are highly effective with a high safety profile. Vaccines are being developed against many of the other bacteria and viruses, and some parasites. Occasionally, a new vaccine has to be withdrawn because of unexpected side effects. Smallpox remains the only infectious disease to have been eradicated. The Global Program to eradicate poliomyelitis initiated in 1988, has unfortunately run into difficulties. A few children immunised with the Sabin oral vaccine fail to clear the virus which can mutate over some years into a pathogenic form and spread rapidly unless large vaccination programs are re-introduced. Of major concern are emerging and re-emerging infectious diseases, especially HIV, for which there is currently no vaccine. Fortunately, new techniques are becoming available making it possible to consider developing vaccines based on inducing strong cell-mediated immune responses to control the agent's replication when antigenic variation in surface antigens (e.g. HIV, influenza) makes classical techniques based on induction of antibody responses less attractive.