What is the most fatal bacterial infection?

The most fatal bacterial infections are those that cause rapid sepsis, meningitis, or plague, with very high mortality if not treated immediately. Historically, the most fatal was bubonic plague caused by Yersinia pestis, which killed 30 to 60 percent of infected people and wiped out one third of Europe in the Middle Ages, though modern antibiotics reduce mortality to about 10 percent if treated early. Today, among common bacterial infections, bacterial meningitis caused by Neisseria meningitidis can kill within hours, with mortality 10 to 15 percent even with treatment and up to 50 percent without treatment, plus causing brain damage. Necrotizing fasciitis caused by Group A Streptococcus, the flesh-eating bacteria, has mortality 20 to 30 percent despite surgery and antibiotics because it destroys tissue rapidly. Sepsis from any bacterial infection that enters bloodstream has overall mortality 20 to 40 percent, making it one of the leading causes of death worldwide, killing an estimated 11 million people annually. Tuberculosis kills about 1.3 million people per year globally, making it the deadliest single infectious bacterium in terms of total deaths, though its individual case fatality is lower at about 5 percent with treatment. In hospital settings, infections with carbapenem-resistant Enterobacterales have mortality 40 to 50 percent because antibiotics fail. Overall, untreated bacterial infections that reach bloodstream and cause septic shock are most rapidly fatal, often killing within 24 to 48 hours, emphasizing need for rapid antibiotics and intensive care.