Billions of pounds have been raised, invested and spent on cancer research over decades, but we still don't have a cure for cancer. We asked experts to explain why this is the case and why we still urgently need to fund more cancer research (why is it hard to cure cancer).

It's important to remember that we've come a long way - overall, cancer survival rates in the UK have doubled in the last 40 years. In the 1970s, only 25 percent of cancer patients survived 10 years or more after diagnosis. Today that figure is 50 percent.

But cancer is a complex disease, and the truth is that we will never find a single cure. Here's why:

Cancer is not just a disease

To understand why we haven't cured cancer, the most important thing to know is that cancer is not a disease. Instead, it's an umbrella term for more than 200 different diseases - which is why we fund research into any type of cancer.

Each broad cancer type has many subtypes that look and behave differently because they differ at the genetic and molecular level. This is because cancer originates in our own cells, so each cancer can be as different and diverse as a person.

There are countless mutations

Behind more than 200 different cancers are countless different genetic mutations. Each cancer is caused by a different set of mutations, and as the tumor grows, more and more mutations accumulate. This means that each tumor has a separate set of mutations, so a drug that works for one cancer patient may have no effect at all on another.

That's why we fund researchers like Dr. Diego Pasini in Italy, whose research project aims to understand why a particular mutation makes some cancers more likely to develop.