Among the reported blood levels of FL-Gal-9 in patients with infectious diseases, we discuss the regulatory roles of FL-Gal-9 in the pathogenesis of human immunodeficiency virus (HIV) infection, HIV co-infected with opportunistic infection (HIV/OI), dengue, malaria, leptospirosis, and tuberculosis (TB). The gene discussed is LGALS9; the disease is leptospirosis.