In normal conversation, "illness", "disease" and "sickness" are interchanged a great deal. However, there is actually some specific difference, especially when used by physicians or any health organization. An illness is a general term that people will use to describe themselves when they do not feel well. They may or may not have been diagnosed by a doctor. A disease is more specific and is determined by a physician or health worker. The term sickness is usually applied if people miss work or cannot function normally in society. In general, "illness" is more general then "disease", which in turn is more general than "sickness".

Sickness is a generic term referring to a condition you experience when do not feel well. Sickness may refer to being nauseated (e.g. motion sickness) or just being unwell because you are ill or tired. Illness is very similar to disease, in everyday speech a disease is typically perceived as a problem more serious or incurable (e.g. a genetic disease). While an illness, on the other hand, is simply the cause of a temporary state of ill health (e.g. the flu).

They're different forms to say someone needs help with their health.