First encountered by the Federation in 2370, the Jem'Hadar are the military arm of the Dominion. They have been the subject of heavy genetic engineering in order to increase their effectiveness as soldiers;  like many Dominion species, the Jem'Hadar are cloned rather than breeding naturally. They are mature within three days of leaving the birthing chamber, allowing for very rapid replacement of casualties.  All Jem'Hadar have an inbuilt ability to 'shroud' themselves, a form of camouflage which effectively hides them from both sensors and the naked eye.  In addition, they are almost completely unaffected by many forms of anti-personnel forcefield.  Jem'Hadar do not need to sleep, eat, or drink - the only substance they require is Ketracel White,  an isogenic enzyme deliberately omitted from their physiology by the Founders. 
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The vast majority of Jem'Hadar never actually see a Founder, and some doubt that they even exist.  have built their service to the shapeshifters into a religion. They literally regard the Founders as living Gods, to the extent that Jem'Hadar have been known to commit suicide if they fail to protect a shapeshifter from harm.  The Vorta, as the representatives of the Founders, are also given immense loyalty by most Jem'Hadar - even when such loyalty is unwarranted.  Absolute obedience is further guaranteed by the Vorta's control of the Ketracel White. 
Most Jem'Hadar die young, in battle - few reach fifteen years of age and any who reach the age of twenty are considered to be honoured elders. No Jem'Hadar has ever reached thirty years of age. 
The most recent change to the Jem'Hadar was the introduction of the 'Alphas' - a breed created in the Alpha Quadrant. The Alphas are regarded - largely by themselves - as being superior to standard Jem'Hadar, but their introduction has led to considerable friction with the older model.