Definition from Wiktionary, the free dictionary
- (pathology, of an illness) Transmitted from one person to another, usually through the air breathed.
- Cancer is not infectious.
- More infectious diseases like the flu are usually less potent.
- (pathology, of a person) Able to infect others.
- Despite feeling better, the patient is still infectious.
- (of feelings and behaviour) Spreading quickly from one person to another.
- Her enthusiasm for work can be really infectious.
- (informal) Memorable and invoking excitement or interest.
- Pop music is more infectious than elevator music.
- (transmitted between persons as illness): catching, contagious
- (able to infect others): contagious
- (spreading quickly between persons): contagious
- (memorable and exiting): catchy, contagious
Derived terms 
transmitted between persons as illness
able to infect others
spreading quickly between persons
informal: memorable and invoking excitement or interest