(virology, dated) virus

(pathology) collagenosis, collagen disease

meeting all three important requirements

(fiction) Hanahaki, an illness affecting those consumed by unrequited love that manifests as the coughing up of flowers, which clears up upon reciprocation. A fictional illness, it has found popularity among both Japanese and foreign online fanfic (especially) artists and authors for its poetic quality.

flat / level / even / smooth

(figuratively) being carefree and unaware of an imminent danger

精神分裂病:

mysterious / strange, bizarre

(dated, potentially offensive) schizophrenia

unequal treaty (treaty forced upon weaker nations by imperialist power)