Signum autem leprae maxime confirmatae apparent in extremitatibus, ut in pedibus, cruribus, et in facie, et praecipue in musculorum corporis consumptione et minoratione. Praeter istas conditiones leprae communes quaelibet species leprae habet suas proprias et speciales.

A demonstrative pronoun or adjective points out "that (particular) one," whereas ille, illa, illud ... tend to mean simply "that," as in "that one over there." 

Forms of iste tend to have a second person implication, and sometimes are mildly negative, as in "...ooh, that girl of yours!"  In the section above, "...istas conditiones leprae communes.. ." would simply be "those common conditions of leprosy...." 

Iste declines just like ille, illa, illud.  Complete the chart below for iste:

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