Chinese character 疟

It means malaria. In Chinese medical terms, 疟 appears in 疟疾 (malaria) and 疟原虫 (the malaria parasite).

Chinese records of malaria-like illness reach back to the Shang dynasty. Classical texts used 瘧 for recurring chills and fever; names like 痎, 痁, 瘴疟, and 鬼疟 did not always mean modern malaria. In 19th- and early 20th-century Chinese writing, 疟疾 competed with 瘴气 (the belief that unhealthy air caused illness) and symptom-based names; by the 1930s–1940s it became standard.

What does 疟 mean when someone sends it?

It points to a disease or medical fragment. Context tells you why it was sent.