There is a difference between being dumb and being ignorant. Everyone is ignorant of something, and most of tons of things. Dumb is a different thing. Don't call yourself dumb just because you don't understand something, that is just a lack of knowledge... easily rectified. Dumb is a human trait that is too many times forever.

There are 7 cancers that are directly caused by viruses. Viruses can be transfered between people. Sometimes that transfer mechanism is via sexual contact. There is nothing more to it than that. This is taken from this OCF web page
http://www.oralcancerfoundation.org/HPV/index.htm

Misunderstanding commonly results when people group viruses and bacteria into the same category of disease. Bacteria and viruses are two completely different organisms, each functioning in different and unique ways. Bacteria are one-celled organisms capable of living and reproducing independently. The major parts of a bacterium cell are; a cell wall, cytoplasm, and a nucleoid. The cell wall protects the cell and gives it shape. The cytoplasm is the fluid inside the cell where cell growth, replication, and metabolism are carried out. The cytoplasm also contains all the components for these actions such as the nucleoid, ribosomes, and plasmids. The nucleoid is the region in the cytoplasm where the strands of DNA are located. Bacteria cells reproduce through a process called binary fission. In this process, a single bacterium cell grows to twice its normal size and splits into two daughter cells. These two new daughter cells are exact copies of the original cell. When bacteria infect a body, they do so by multiplying inside the organism and producing toxins that infect surrounding cells or tissues. But very few bacteria are harmful. 99% of the bacteria found in the body are helpful, and most are actually necessary for life. Harmful disease-causing bacteria can be eliminated or killed with antibiotics.

Viruses on the other hand, exist and function in an entirely different manner than bacteria. They are not an independent organism. When a virus is alone it is not metabolically active like bacteria; it needs a host cell to function, and in which to reproduce. A virus is composed of only a protein capsule that encloses its DNA or RNA. When a virus comes in contact with a cell, it is capable of inserting its genetic material into that host cell and taking over the functions of that cell.

OK so here's the take away from all this, and when you get it, you are no longer ignorant. Taking over the functions of the cell, can be things like destroying some of the important components of the cell such as the genetic information that it carries and which control the cells functions. If it kills gene p53 for instance, that makes the cell immortal. (HPV does this) If it destroys rb, which is the tumor suppressor gene, the cell can no longer signal the immune system that it is damaged and attract our various immune responses to come and destroy it. HPV does this too. Now we have a cell that is on its way to becoming malignant or cancerous. When that cell duplicates itself, it makes copies that already have these changes in them. A bunch of these begin to collect together as more and more of them duplicate and you have a group of them. This would be what we think of as a tumor. So while this is a very simplified description of what happens, you can see that something as simple as a virus can become something deadly.