The National Institutes of Health (NIH) is attempting to develop a new, safer vaccine against smallpox. This is part of an ongoing attempt to protect the nation against the threat of bioterrorism and bioweaponry--that is, the use of weaponized pathogens. Similar research is being conducted to protect the populace against other diseases, such as the plague.
Various species of monkeys have been used to test the efficacy of the proposed vaccine (modified vaccinia Ankara, or MVA). Monkeys are inoculated with MVA, then exposed to the monkeypox virus, which closely resembles smallpox. The researchers then determine if the vaccine provides any protection against the virus. Unfortunately, this does result in the monkeys' deaths.
It would clearly be unethical to test the vaccine on humans in the same fashion. (I have a hard enough time thinking about these poor simians.) Fortunately, the ELISPOT technique allows immune response monitoring to be performed ex vivo; that is, without injecting any pathogens into a human host.