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This week, America lost one of its greatest innovators to what was likely a recurrence of the pancreatic cancer he had battled for years. Steve Jobs, the co-founder and former CEO of Apple Inc, was just 56 when he died Wednesday.

It’s hard to fathom that a man who was at the forefront of the latest technological developments, and who likely had access to some of the best medical care in the world, could succumb to cancer – especially since his 2004 announcement treatment had worked, and a successful liver transplant in 2009.

Doctors speculate that his cancer likely returned and maybe even spread to his lymph nodes or liver.

The truth is, there are a number of cancers – pancreatic cancer, included – that are still virtually undetectable, even after all the great strides we have made in medicine over the years.

What’s worse, is that these cancers can develop unexpectedly and “disguise” themselves as other health conditions.

Pancreatic cancer

Pancreatic cancer, which affects about 44,000 Americans each year, can usually only be controlled if it is found before it spreads – and it is very hard to find in some cases. That’s because there aren’t any noticeable signs or symptoms in the early stages of pancreatic cancer.

When signs, such as jaundice, back/abdominal pain and weight loss, finally do begin to present themselves, they can be confused as symptoms of a number of other illnesses. Additionally, the pancreas is hidden behind other organs such as the stomach, small intestine, liver, gall bladder and spleen.

Kidney cancer

Like pancreatic cancer -- kidney, or renal cell cancer -- is hard to detect because there are few symptoms in the early stages of the disease, which affects 54,000 people in the U.S. per year. One of the earliest warning signs is discolored urine, or urine that has a high blood cell count. A person might also experience back pain and weight loss.

There are certain risk factors for kidney cancer, such as smoking, obesity or high blood pressure, but the cancer also develops in many people who do not have any of these risk factors, and doctors still cannot explain why this happens.

Gastric cancer

Gastric cancer, or stomach cancer, is diagnosed in 21,000 Americans per year, and typically only half of them survive. Since symptoms of gastric cancer often do not appear until the disease is advanced, only one in five cases is found at an early stage before it the cancer spreads to other areas of the body.

Signs of the cancer can include poor appetite, weight loss, abdominal pain, discomfort above the navel, a sense of fullness in the upper abdomen after small meals, heartburn, indigestion, nausea, vomiting and abdominal swelling.

Certain kinds of leukemia

Finally, with leukemia, the difficulty of detection depends on what type a person develops. The American Cancer Society lists four types of leukemia: Acute lymphocytic, chronic lymphocytic, acute myeloid and chronic myeloid. Combined, the four kinds of the disease affect 45,000 Americans annually.

Chronic forms of leukemia develop slowly over a period of years, while acute forms develop rapidly and require immediate treatment for the person to survive longer than a few months.

While there is less time to treat acute leukemia, the symptoms are more visible. They include fatigue, shortness of breath, pale skin, fever, prolonged bleeding from minor cuts, unexplained bruises, and tiny, red spots under the skin. Chronic leukemia, on the other hand, is typically only identified by a blood test taken during a routine exam or during testing for an unrelated problem.

Unfortunately, cause of leukemia is still largely unknown. But researchers believe it may be a combination of environment and genetic factors.

All of these cancers listed above – pancreatic, kidney, gastric and leukemia – can be diagnosed through physical exams and medical tests such as CT scans, MRIs, blood tests and ultrasounds (though the tests vary according to the type of cancer).

The best way to protect yourself is to know your body and see a doctor right away if you notice any significant changes.

Steve Jobs allowed us to see the world through the eyes of a true technological visionary. But let us also learn from his courageous fight against this devastating disease to be proactive in the fight against it.