In "274", it's revealed that one of the base's other labs has been testing an anti-viral agent against every known virus, including smallpox: the one human disease which has been driven completely extinct, save a handful of laboratory samples. The revelation that the base has such samples is scary enough ... but to test a cure for smallpox, they'd presumably have to infect some research subjects and see if the cure helped them. Smallpox was exclusively a human disease; it can't infect other animals. So if they really tested their cure on smallpox, they had to have used human test subjects ... and the cure, itself, kills 75% of those treated with it.
Though that well may be what really happened given Hatake's evasiveness and Balleseros' suspicions, That Other Wiki does mention other animals, (like primates) have been infected in lab settings.
