HLB was originally thought to be a viral disease, but was later discovered to be caused by bacteria, carried by insect vectors. HLB infection can arise in various climates and is often associated with different species of psyllid insects. For example, citrus crops in Africa become infected under cool conditions as the bacteria are transmitted by the African citrus psyllid "Trioza erytreae", an invasive insect that favors cool and moist conditions for optimal growth. Citrus crops in Asia, however, are often infected under warm conditions as the bacteria are transmitted by the Asian citrus psyllid "Diaphorina citri".