Dementia paralytica results from late-stage syphilis invading the central nervous system and was a fatal, untreatable condition in the early 20th century. Physicians had noticed that high fevers sometimes suppressed infections, and Wagner-Jauregg set out to test this observation systematically. He chose malaria (mainly Plasmodium vivax) as a controllable fever-inducing agent and inoculated patients, producing repeated spikes of about 41 °C. After the fever coursed, he eradicated the malaria with quinine and observed long-term remission in roughly 30 % of patients—a remarkable outcome for that era. The work introduced the concept of “pyrotherapy” to psychiatry and remained a standard treatment until antibiotics became available. It also changed public perception by demonstrating that some severe mental illnesses could, in fact, be medically treated.

Wagner-Jauregg’s work was groundbreaking because it translated the 19th-century observation that fever boosts immunity into a controlled clinical intervention. The target condition, dementia paralytica, is a chronic meningoencephalitis caused by Treponema pallidum; contemporary treatments such as mercurials and Salvarsan showed poor efficacy. Plasmodium vivax was chosen as the fever source because of its relatively low lethality and its predictable, high-amplitude temperature spikes. Clinical trials between 1917 and 1921 involving 223 patients reported symptomatic improvement in roughly 60 % and substantial remission in 20–30 %. Although the exact mechanism remains debated, increased cytokine production and augmented cerebral blood flow during hyperthermia are thought to inhibit treponemal replication. The therapy could be administered inside psychiatric hospitals, facilitating a shift from custodial care to active medical treatment—a major social advance. Nevertheless, complications such as heat-related neurological damage and hemolysis from malaria occurred, highlighting the need for risk management. After mass production of penicillin in the 1940s, malariotherapy quickly fell out of favor, yet it retains historical significance as a precursor to biological psychiatry. Modern researchers in hyperthermia and immunomodulation still draw lessons from this classical experiment about the interplay among inflammation, heat, and the immune system.

From the early 1900s Wagner-Jauregg investigated pyrotherapy for psychiatric disorders and on 14 June 1917 reported the first case of deliberate malaria inoculation. He used fresh blood verified for Plasmodium vivax parasitemia, induced approximately four febrile paroxysms, and continuously charted the temperature curve. Therapeutic endpoints included clinical symptom scores and conversion of the CSF Wassermann reaction to negative; 32–35 % achieved full remission and >40 % showed marked improvement. Although contemporary statistics were rudimentary, Wagner-Jauregg claimed significance comparable to p < 0.05, and later meta-analyses report an odds ratio of about 2.5. Pathological studies demonstrated activation of astroglia and reduction in treponemal antigen load after heat stress, with hypotheses involving heat-shock protein induction. Standard protocol comprised oral quinine 4 × 0.25 g/day; severe febrile crises were managed by cold sponging and intravenous phenobarbital. By the late 1920s attenuation of Plasmodium strains reduced the case-fatality rate from an initial 15 % to below 3 %. Penicillin’s advent reversed the benefit-risk ratio, and WHO removed malariotherapy from recommended treatments in the 1950s. Nonetheless, the study is credited with inaugurating psychoneuroimmunology and influencing biological models of mental illness. Recent work on IL-6/TNF-α-induced behavioral changes is re-examining Wagner-Jauregg’s clinical observations as foundational data.