Your question is a very complex one. There are lots of diseases out there that are very dangerous for humans. Some diseases are capable of being spread very easily. These diseases are called infectious disease. This means it can be passed between people. An example of a very serious infectious disease is human immunodeficiency virus (HIV). Other diseases cannot be passed between people (for example, heart disease). These diseases may be caused by specific mutations in a person's DNA or may be caused by environmental factors (for example, poor diet, smoking).

There are many factors that make a particular disease dangerous: 1) How easily does this disease spread from person to person? 2) What is the likelihood that a person with this disease will become extremely sick or die? 3) What is the likelihood someone will contract this disease? For example, there are very deadly diseases (such as a Ebola), which are nearly always fatal if one gets it. However, unless you live in a specific region of the world the chances you would catch this disease is very small. There are other diseases that are less fatal but spread very quickly and cause a lot of people to get sick (such as the flu). As you can see it would be hard to pinpoint the most dangerous disease.

Here are some links to websites that describe very serious diseases:

I'm not sure you could classify any particular disease as being the most dangerous for human beings, it would also depend on what you consider dangerous. Is it that a disease can kill rapidly, or is it that it can infect people very quickly, or maybe the combination of the two - a rapidly infecting killer. Recently, there have been many outbreaks of various diseases that spread rapidly and kill many people. For example, the Avian flu, the swine flu, and SARS. In previous centuries these could have been the bubonic plague, small pox, tuberculosis, or measles. In general, I believe that diseases that can easily be transmitted from person to person, have long lives out side of the body (can linger on a surface for a day +), and can easily shut down human organs or systems (nervous system, cardiac system, etc.) would be something that is the most dangerous for human beings.

Thankfully, over the last century and a half, scientists have been working on vaccinations that keep us from becoming infected with some of these diseases. For example, smallpox (the first vaccination was made for this in the 1796 by Edward Jenner) was eradicated (killed off/no longer exists) only recently in 1980. We have many different vaccinations that we are given when we are young to keep from acquiring the disease. You can read about some of these disease at the website for the Center for Disease Control and Prevention (CDC):