I’m not sure if I fully understand the question but here goes…
The disease which kills the most people worldwide every year is heart disease, killing over 7 million people every year according to the World Health Organisation (WHO). Other diseases which kill millions of people every year include strokes, bronchitis and pneumonia, diarrhoeal diseases, HIV/AIDS, cancer, tuberculosis and diabetes. The next biggest killer each year is road traffic accidents, which kills 1,210,000 people every year.
If you are interested in poisons, the deadliest poison known to man is called the Botulinum toxin, which is produced by the bacterium Clostridium botulinum. The staggeringly small amount of 90 nanograms (a nanogram is one billionth of a gram) is enough to kill an average sized person, meaning a teaspoonful could kill over 55.5 million people! The toxin works by interfering with nerve function, preventing muscles from working properly, causing the body to go limp with paralysis and death normally occurs from breathing difficulties.
Botulism (the disease caused by the toxin) is rare. According to the Centre for Disease Control in the USA, about 145 cases are reported each year and 95% of people survive with breathing assistance and intensive care. These cases are from food (normally canned food that hasn’t been produced properly), from wound infections (Clostridium spores can get into the air) and in infants who breath in the spores and aren’t strong enough to fight the infection (honey can contain the bacteria that causes Botulism so infants younger than 12 months should not be given honey, but it is perfectly safe for everyone else).
Remarkably, the Botulinum toxin has medical and even cosmetic uses, but obviously in very small doses! You may have heard of “Botox” which is used to reduce wrinkles by relaxing the muscles in the face. Well this is the very same toxin BOtulinum TOXin that causes Botulism and every drop of it used around the world is made by a company called Allergan in Westport, Co. Mayo! Apart from ironing out wrinkles, Botox is also used to treat migraines, overactive bladders and even excess sweating.
I hope that answers your question (and maybe more) but if it doesn’t please let me know by commenting below.