Many types of bacteria can be fatal to humans and it is hard to say which one is the deadliest. But before I tell you more about fatal bacteria let me first tell you that the vast majority (over 99%) of bacteria is not only harmless but is actually considered helpful.

>For those bacteria that are fatal the "deadliest" can be considered in two ways. If "deadliest" means that one dies quickly then the bacteria that cause meningitis (this is an inflammation of linings in the brain and nerves) are most fatal (within 24 hours if not treated). Other bacteria can be deadly to many people like the bubonic plague or cholera. They are fatal within an few days if they are not treated with antibiotics. Most of the bacterial infections can be treated with antibiotics but we are seeing more and more strains that are resistant due to overuse of antibiotics. In poor countries people often can not afford medication.

The last group of bacteria I want to mention are the ones that make and secrete toxins. The bacteria themselves are not the problem but their toxin can be fatal. One example are species of the bacteria Clostridium. They secrete toxins causing paralyses and the decease is called tetanus. In that case killing the bacteria after it has secreted the toxin does nothing to reverse the effect of the toxin. That is why you need to make sure you get your tetanus shots every ten years. I hope this answers your question. Stay curious ... and healthy!