The aim was to study the stimuli responsible for triggering and sustaining the plasma concentration of the inflammatory marker interleukin-6 (IL-6) in patients with a first myocardial infarction before the age of 60 and healthy control subjects matched for age and sex. The plasma IL-6 concentration, antibodies against Chlamydia pneumoniae, cytomegalovirus, Epstein-Barr virus, Helicobacter pylori, herpes simplex type 1 and 2, and genotype for the IL6-174 G>C single-nucleotide polymorphism were determined 3 months after the acute event. The results showed that patients had higher IL-6 levels than control subjects, whereas there were no differences regarding individual or total number (pathogen burden) of positive antibody tests against the different pathogens or IL6 genotype distribution. The plasma IL-6 concentration was associated with the number of positive antibody tests in patients and control subjects, whereas patients irrespective of IL6 genotype had increased IL-6. Multivariate analysis, including traditional coronary heart disease risk factors, antibodies against pathogens, and IL6 genotype, explained 17% of the variation of the plasma IL-6 concentration. Neither pathogen burden nor IL6 genotype did contribute to the variation of plasma IL-6 levels, whereas smoking, body-mass index, hypertension, case-control status, and age were determinants of the plasma IL-6 concentration.