A sign of something being wrong is something that can be seen by someone else, perhaps a loved one, or a doctor, nurse, or other health care professional. For example, fever, fast breathing, and abnormal lung sound heard through a stethoscope may be signs of pneumonia.
Having one sign or symptom may not be enough to figure out what’s causing it. A doctor can get a better picture of the illness. Sometimes, a patient’s signs and symptoms still don’t give the doctor enough clues to be sure what’s causing the illness. That's when medical tests might be needed.

How do cancer cause signs and symptoms?

Cancer is a group of diseases that can cause almost any sign or symptom. The signs and symptoms will depend on where the cancer is, how big it is, and how much it affects the organs or tissues. If cancer has spread, signs or symptoms may appear in different parts of the body.
As cancer grows, it can begin to push on nearby organs, blood vessels, and nerves. This pressure causes some of the signs and symptoms of cancer. If the cancer is in a critical area, such as certain parts of the brain, even the smallest tumor can cause symptoms.
But sometimes cancer starts in places where it won’t cause any signs or symptoms until it has grown quite large. Cancers of the pancreas, for example, usually don’t cause symptoms until they grow large enough to press on nearby nerves or organs (this causes back or belly pain). Others may grow around the bile duct and block the flow of bile. This causes the eyes and skin to look yellow (jaundice). By the time pancreatic cancer causes signs or symptoms like these, it’s usually in an advanced stage. This means it has grown and spread beyond the place it started, the pancreas.
Cancer may also cause symptoms like fever, extreme tiredness (fatigue), or weight loss. This may be because cancer cells use up much of the body’s energy supply, or they may release substances that change the way the body makes energy from food. Cancer can also cause the immune system to react in ways that produce these signs and symptoms.
Sometimes, cancer cells release substances into the bloodstream that cause symptoms that are not usually linked to cancer. For example, some cancers of the pancreas can release substances that cause blood clots in the veins of the legs. Some lung cancers make hormone-like substances that raise blood calcium levels. This affects nerves and muscles, making the person feel weak and dizzy.
How are the signs and symptoms helpful?
The treatment works best when cancer is found early, whilst it’s still small and is less likely to have spread to other parts of the body. This often means a better chance for a cure, especially if cancer can be removed with surgery.
Sometimes people ignore symptoms. Maybe they don’t know that the symptoms could mean something is wrong. Or they might be frightened by what the symptoms could mean and don’t want to get medical help. Maybe they just can’t afford to get medical care.
Some symptoms, such as tiredness or coughing, are more likely caused by something other than cancer. Symptoms can seem unimportant, especially if there’s a clear cause or the problem only lasts a short time. In the same way, a person may reason that a symptom like a breast lump is probably a cyst that will go away by itself. But no symptom should be ignored or overlooked, especially if it has lasted a long time or is getting worse.
Most likely, symptoms are not caused by cancer, but it’s important to have them checked out, just in case. If cancer is not the cause, a doctor can help figure out what the cause is and treat it, if needed.
What are some general signs and symptoms of cancer?
You should know some of the general signs and symptoms of cancer. But remember, having any of these does not mean that you have cancer, there are many other things that cause these signs and symptoms, too.
If you have any of these symptoms and they last for a long time or get worse, make sure you visit a doctor to find out what is happening.
Unexplained weight loss
Most people with cancer will lose weight at some point. When you lose weight for no known reason, it’s called unexplained weight loss. Losing 10 pounds or more without trying may be the first sign of cancer.
Fever
Fever is very common with cancer, but it more often happens after cancer has spread from where it started. Almost all people with cancer will have fever at some time, especially if cancer or its treatment affects the immune system.
Fatigue
Fatigue is extreme tiredness that doesn’t get better with rest. It may be an important symptom as cancer grows.
Pain
Pain may be an early symptom with some cancers like bone and testicular.A headache that does not go away or get better with treatment may be a symptom of a brain tumor. Most often, pain due to cancer means it has already spread from where it started.

Signs and symptoms of certain cancers

Along with the general symptoms, you should watch for certain other common signs and symptoms that could suggest cancer. Again, there may be other causes for each of these, but it’s important to see a doctor about them as soon as possible especially if they last a long time.
Change in bowel habits or bladder function
Long-term constipation, diarrhea, or a change in the size of the stool may be a sign of colon cancer. Pain when passing urine, blood in the urine, or a change in bladder function could be related to bladder or prostate cancer.
Sores that don't heal
Skin cancers may bleed and look like sores that don’t heal. A long-lasting sore in the mouth could be oral cancer. This should be dealt with right away, especially in people who smoke, chew tobacco, or often drink alcohol.
Unusual bleeding or discharge
Unusual bleeding can happen in early or advanced cancer. Coughing up blood may be a sign of lung cancer. Blood in the stool (which can look like very dark or black stool) could be a sign of colon cancer. Cancer of the cervix can cause abnormal vaginal bleeding.
Thickening or lump in the breast or other parts of the body
Many cancers can be felt through the skin. These cancers occur mostly in the breast, testicle, lymph nodes (glands), and the soft tissues of the body. A lump or thickening may be an early or late sign of cancer and should be reported to a doctor.
Nagging cough or hoarseness
A cough that does not go away may be a sign of lung cancer.
The signs and symptoms listed above are the more common ones seen with cancer, but there are many others that are not listed here. If you notice any major changes in the way your body works or the way you feel – especially if it lasts for a long time or gets worse – let a doctor know.