Influenza, commonly known as the 'flu', is an infectious disease of birds and mammals caused by influenza viruses. Cancer is defined as an uncontrolled division or proliferation of cells without any differentiation. It is a non-infectious disease caused by the agents called carcinogens.
Smallpox is a serious, highly contagious and often life threatening disease characterised by a rash and blisters on the face, arms and legs. It is caused by the Variola virus. Allergy is the exaggerated response of the immune system to certain antigens in the environment (pollen, dust, mites, molds, cloth fibres, animal hair, etc). It occurs due to the release of chemicals like histamine and serotonin from the mast cells. It is non-infectious response.

──────────────────────────────────────────────────────────────────────────────────────────