The early symptoms of a disease are not detectable by modern machines. Suppose you get a hair in your eye. No amount of x-ray can detect that, but you can feel it. Suppose you have a 1mm tumor in your liver. There is no way that an ultra sound or CT scan will pick it up. The detectable mass can be 2-3mm in diameter. There can be millions of cancer cells in this 1mm mass.
Cancer does not develop in a short time. It starts with unregulated mutations. If the person does not cure the problem it develops in 3-4 years into hyperplasia. If the excesses with the body keep on it develops into dysplasia in further 3-4 years. If still no corrective measures are taken it develops into in-situ cancer in 3-4 years. This in-situ cancer is detectable when it reaches the size of 2 mm or more.
All this time the patient has been complaining of different problems. Headaches, menstrual problems, constipation, insomnia, fatigue, cough etc. etc. Since machines were unable to detect this life-threatening problem, the patient ended up with cancer.
Homeopaths do not wait for a disease to progress to an observable level by machines. They observe the objective and subjective symptoms. We can feel quantum changes in our bodies which simply are not possible to be picked up by machines. Our brain contains 95-100 billion neurons. It can detect signals which are not detectable by any other media.