There have been many and it would depend on what time frame you are referring to. The Black Death or Bubonic Plague, at one time, did infect at least that many in the world. Based on WHO's statistics, the combination of AIDS, malaria, and tuberculosis is responsible for half (approximately 5 million) of all infectious-disease deaths each year. Here are the five most common Infectious Diseases today according to WHO: Hepatitis-B According to current statistics, hepatitis-B is the most common infectious disease in the world, affecting some 2 billion people -- that's more than one-quarter of the world's population. This disease, which is characterized by an inflammation of the liver that leads to jaundice, nausea, and fatigue, can lead to long-term complications such as cirrhosis of the liver or even liver cancer. The concern is primarily for those who carry the chronic form of the disease, which is estimated to be about 350 million people. Malaria, a mosquito-borne disease that tends to affect children the most in tropical and subtropical climates, affects more than 500 million people annually and results in anywhere between 1 million and 3 million deaths. Behind hepatitis-B, it appears to be the second most-common infectious disease, and it certainly is one of the most deadly on an annual basis. Hepatitis-C is a less common and less severe form of hepatitis, but it almost always develops into a chronic, not acute, condition, unlike hepatitis-B. Although only 3 million to 4 million new cases are reported each year, some 180 million people worldwide suffer from this chronic condition, which can lead to liver cancer or cirrhosis of the liver over time. Dengue fever, affects approximately 50 million people each year. Dengue is most common in Africa and Asia and thankfully occurs in only mild to moderate forms, which can cause high fever, severe headaches, and joint and muscle pain, but rarely leads to the death of the infected patient. Amazingly enough, even though this infectious disease affects some 50 million people annually, there isn't a specific drug designed to treat dengue fever. TB is caused by a bacteria found in the lungs that can cause chest pain and a bad cough, as well as lead to a number of other nasty side-effects. According to WHO, it's also the second-leading global killer behind AIDS as a single infectious agent. The majority of TB-associated deaths (95%) occur in low- to middle-income countries where TB awareness and prevention simply aren't where they need to be. The good news is that TB death rates on a global basis are falling; however, there were still 8.6 million new cases of TB reported last year, and roughly one-third of the world's population carries a latent form of TB, meaning they've been infected but aren't ill and can't transmit the disease yet. ~By Sean Williams~