Well, it depends on how you define "deadliest." If you mean what kills the most people, the deadliest disease in the US is nicotine addiction. Smoking kills more people than any other disease in America by causing, or helping to cause, heart disease, high blood pressure, several types of cancer, and a variety of breathing diseases.

Worldwide, tuberculosis or malaria kill the most people per year. About 300-500 million people get malaria every year, and about 1% of them die. (1% seems pretty small doesn't it? But how many people is that?) We don't hear about it a lot in the United States because it is mostly found in tropical areas. Malaria is caused by a microscopic parasite. It lives inside red blood cells (RBCs carry oxygen). Mosquitoes carry the parasite from one victim to another. Victims of the disease suffer from severe cycles of fevers and chills.

Let me use this as an example of one of the ways science helps people. By using the scientific process, people discovered the cause, treatment, and prevention of malaria . In fact, the word "malaria" means "bad air" because people used to think that bad air caused the disease. If you don't know what causes a disease, it's hard to stop yourself from getting it. Now that we know, we have medicines to stop people from getting malaria, medicines to cure it, and ways to avoid getting bitten, like mosquito netting, repellent, and poison. Communities can also take simple steps like getting rid of trash that holds water (old tires, open cans, etc.) because mosquitoes lay their eggs in them. Malaria is still common because many people cannot afford the drugs or prevention programs. This is where science alone is not enough to solve the problem. What would help us protect more people from malaria?

The most common disease listed by the World Health Organization is diarrhea. There are about 4 billion cases a year. (How many people are there in the world?) This doesn't really count as one disease because diarrhea can have a lot of causes. About 2 billion people have tuberculosis.

Why would it be difficult to say what the rarest disease is?

This is a useful and serious site which you may visit to learn more about diseases in the world:
World Health Organization