The Human-San'Shyuum alliance was an alliance between the two species during the reign of the Forerunners.
Even before they united their military and scientific strength against the Forerunners, the Humans and San'Shyuum regularly traded produce, items, and technology with each other. One such example is the Pheru, an animal domesticated and used as a pet.
Human-Flood war Edit
The San'Shyuum, like the humans, had used the powder on the Pheru to make them more "exotic", with an active black market in them. When the Pheru became aggressive due to the "infection", they killed their favorite pets, much to their dismay.
Although San'Shyuum were infected by the "plague" during the Human-Flood war, it is apparent that they were not as weakened by it as the humans were, since they did not need any uninfected worlds like the humans did.
Human-Forerunner war Edit
During the Human-Forerunner war the humans were already fighting Forerunners when they allied themselves with the San'Shyuum, combining their technology, weapons, and knowledge. It was this alliance that made humans formidable opponents for the Forerunners, inflicting many casualties on them. Despite both species being incredibly powerful, humanity was already weakened fighting a war with the Flood. Eventually, the alliance was destroyed in the Battle of Charum Hakkor, with human resistance completely crushed and the San'Shyuum finally surrendering.
The humans, being cruel and fighting to last man, were 'deevolved', with the The Librarian being the only reason for their survival. The San'Shyuum, more willing to accept defeat, were quarantined to their native system. For obvious reasons, it was forbidden for the humans (or rather, what was left of humanity) to ever meet the San'Shyuum.