Cancer

It only takes one rogue cell to cause cancer. All cells are programmed to kill themselves if they detect a problem within their systems. If they sense a virus or mutation, they destroy themselves completely. But some cells develop a mutation that cancels out that defense mechanism, which is called Apoptosis. Of the 100 billion cells created in the human body each day, at least 100 thousand mutate and perform apoptosis.

At first, a cell that mutates so as not to perform apoptosis will be harmless. Through mitosis it would replicate itself and it's copies would have that same mutation. But eventually, one of those cells would form it's own mutation, and because of the original cell's mutation, it wouldn't kill itself. The new mutation might be something like replicating itself faster than normal. At this point the cells are still harmless. The cell with two mutations would continue to replicate itself. But the odds of mutation in cells are high, and without apoptosis, the mutations wouldn't be dealt with. Soon cells might develop a mutation causing them to secrete acids and waste that kill the cells around them, or maybe it would be a sensory mutation that makes the cell think that other cells are viruses. Mutated cells can even cause marks or rashes on the skin.

Mutated cells are called cancer cells, and you can imagine that they would be very hard to eliminate. It would be difficult for a drug to seek out the mutated cells and not kill the healthy one. For a virus medicines and drugs would simply kill all organisms in that body that match a viruses description. For cancer cells, a cure would need to cleanse the body of mutated cells that don't match any description.

Viruses

To understand viruses you must first have a basic knowledge of what DNA and RNA are. DNA are double-stranded strips of code stored in the cell's nucleus. They split themselves into two single-stranded strips called RNA, then they leave the nucleus. The RNA codes to proteins, which make up the structure of a human being. On the left is a picture of DNA before it splits itself.

Viruses are capsules that store their own RNA. They infiltrate cells and replace the organisms RNA with their own. The cell, thinking the RNA is their own, will then use it's resources to code the RAN into more viruses. Then, depending on the type of virus, one of two thins will happen. The virus will either stay and wait for other viruses to form, or leave immediately to infect other cells. If it waits, the cell will eventually host a large amount of viruses. The viruses will then release proteins that will cause the cell to kill itself. This is a process called lysing. If the virus leaves immediately, or buds, it will push against the cell wall until a piece of it breaks off surrounding the cell. This will help the virus infect other cells because the broken cell wall disguises the virus as just another cell.

There are possibly hundreds of unique ways viruses can infect a cell or bacteria, with more being discovered every week. But only three ways are most commonly spoken of. The first is for a virus that has acquired a piece of a cell wall to push itself toward the cell. It's membrane will attach to the cell's, fusing the two together and allowing the virus to plant it's RNA. The second technique is to attach a tube-like extension of the virus' body to the cell wall and inject it's RNA. This works much like a tetanus shot. Only specific types of viruses can perform the third method. Similar to the Greeks Trojan Horse, the virus disguises itself as a nutritious particle that the cell needs to consume. From there it's obvious to see how this method works.