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Endometritis is a disease in which bacteria enter the uterus and cause inflammation of the lining.
Literal
Endometritis [topic-は], bacteria [subject-が] uterus-inside [into-に] enter, lining [at-に] inflammation [object-を] cause disease is.
A clinical definition. The sentence structure is definitional: ~は~病気である (here shortened to である dropped in favor of a simpler construction), where a long modifier clause sits before 病気 ('disease'). The modifier chains two verbs with the written connective form: 入り (the stem form of 入る as a linking verb) + 起こす. ~を起こす is the standard collocation for 'cause (a symptom/condition)' — 炎症を起こす ('cause inflammation'), 事故を起こす ('cause an accident'). 子宮内膜炎 is a Sino-Japanese medical compound — 子宮 + 内 + 膜 + 炎.