Chloroquine (closely related to the naturally-occurring Quinine which has been used for centuries to treat and prevent malaria) was discovered in 1934 by Hans Andersag. and is on the World Health Organization's List of Essential Medicines.

Chloroquine was, until recently, the most widely used anti-malarial. It was the original prototype from which most methods of treatment are derived. It is also the least expensive, best tested and safest of all available drugs. The emergence of drug-resistant parasitic strains is rapidly decreasing its effectiveness; however, it is still the first-line drug of choice in most sub-Saharan African countries.[…] Chloroquine is a 4-aminoquinolone compound with a complicated and still unclear mechanism of action. It is believed to reach high concentrations in the vacuoles of the parasite, which, due to its alkaline nature, raises the internal pH. "

Source : Wikipedia

The drug has also been used to treat Rheumatoid Arthritisplugin-autotooltip__plain plugin-autotooltip_bigRheumatoid Arthritis

Rheumatoid Arthritis (RA) is a chronic severe inflammatory disease believed to affect around 1% of the global population. It primarily affects joints,which typically become warm, swollen and painful. Pain and stiffness often worsen following rest. Most commonly, the wrists and hands are involved, with the same joints typically involved on both sides of the body. and Lupusplugin-autotooltip__plain plugin-autotooltip_bigLupus

"Systemic lupus erythematosus, also called SLE or lupus, is a disease of the immune system that causes the immune system to attack tissues in various parts of the body. The cause of lupus is unknown. Finding the cause is the object of major research efforts., in these cases, the mechanism of action is also unknown,

NOTE : It has also been tested with regard to the SARS-CoV-2 virus (COVID 19). In this case its effectiveness is disputed, and in 2021 the US NIH recommended against its use for treating COVID patients ref. (archived)