The terms illness and disease are often used synonymously but they do have different meanings. The term illness, is commonly used to describe how the patient feels, namely whether or not they feel unwell. A disease on the other hand refers to an abnormal state of the body, typically seen by the appearance of signs and symptoms of a particular type.
Diseases may be the result of an invasive organism such as a virus or bacteria, or dysfunction brought on by internal causes such as metabolic disorder, neurological problems, and autoimmune conditions. Diseases are typically distinguished from injuries which are the result of trauma. Diseases need not be restricted to the physical body but can also affect mental states though there can be very negative connotations to the use of the term.
It is perfectly possible for patients to be suffering from a disease but not to feel unwell and therefore they do not report an illness. Sometimes, it requires laboratory tests to even identify that a patient is suffering from a disease since the symptoms are not noticed by the patient. In the case of carriers, they can even have the disease but not suffer from the normal symptoms at all.
Where a disease has a long latency period, humans can carry the disease and transmit it without showing signs of symptoms. The HIV virus is an example in which the host remains apparently healthy for long periods of time, showing no symptoms, but able to transmit the disease to others. Asymptomatic carriers are also known in diseases such as typhoid and tuberculosis. There are also genetic carriers in which they possess genes which are not expressed, or where the genes are recessive. In those cases, the following generation risk the appearance of the disease if both parents carry the recessive genes. Haemophilia is one such disease.
In the case of flu, patients contracting the disease remain asymptomatic for up to three days and therefore don't suffer from the illness. In that time, they can transmit the disease to others before they develop the symptoms. In addition, if you have already had an infection of a particular strain of the virus, you may be infected with it but not suffer the symptoms because your immune system already has the antibodies. You could though still transmit the disease to someone else.
There is a social difference in the usage of the terms disease and illness as well. The term disease often has a very negative overtone and since historically disease and poverty were closely associated, there were moral associations with the term. In Victorian England, there was a belief that poverty brought on disease, and rather than understanding the causes of poverty, the poor were often blamed for the outbreak of diseases.
A more modern term, condition, has largely replaced the use of the term disease as it clearly excludes blame. Patients are not responsible for their medical condition whereas there possibly persists a hidden prejudice associated with the term disease.