I realize that PMR has, currently, no known single cause. I used AI (Perplexity) to answer the questions, "What causes PMR?" Here is what it said.
Polymyalgia rheumatica (PMR) does not have a single known cause; current evidence points to a mix of genetic susceptibility, aging of the immune system, and environmental triggers such as infections or (more rarely) vaccines and other inflammatory stressors. It is best understood as an immune‑mediated inflammatory disorder rather than something caused by muscle overuse or a mechanical problem.

What is known vs unknown

Experts agree that the exact cause of PMR is unknown, so there is no single confirmed trigger that explains all cases. PMR is thought to arise when an aging immune system becomes dysregulated in genetically susceptible people, leading to inflammation around shoulders, hips, and other proximal structures.

Genetic and age factors

Family clustering and specific immune‑related genes (such as certain HLA class II types, especially HLA‑DRB1*04) suggest a genetic predisposition to PMR. The condition occurs almost exclusively in people over 50, with peak onset in the 70s, indicating that age‑related immune and hormonal changes contribute substantially.

Immune and autoimmune mechanisms

PMR is an immune‑mediated inflammatory disease where cytokines such as interleukin‑6 (IL‑6) are elevated, and patients typically have raised inflammatory markers like ESR and CRP. The immune profile shows changes in T cells and B cells consistent with an autoimmune‑type process, but there is no single characteristic autoantibody, so it is often described as immune‑mediated rather than classic autoantibody‑driven autoimmunity.

Environmental and infectious triggers

Several lines of evidence suggest that environmental factors can trigger PMR in predisposed individuals, including seasonal clustering of new cases and associations with infections. Proposed infectious triggers have included respiratory pathogens such as Mycoplasma pneumoniae, parvovirus B19, and other respiratory viruses, but none has been definitively proven as “the” cause.

Vaccines, comorbid disease, and other associations

Case reports describe PMR or PMR‑like syndromes occurring after influenza or other vaccinations, possibly as part of a broader autoimmune/inflammatory reaction, though this remains uncommon and causality is not firmly established. PMR is closely linked with giant cell arteritis and has reported associations with conditions like diverticulitis and, rarely, underlying malignancy, reinforcing the idea that systemic immune and inflammatory milieu, rather than a single external agent, drives the disease.