Not all examples below are from my field, but just some random thoughts from a variety of fields.
"ab + c ==> ab + cb"This expression is true for many cases where 'b' is contagious and/or where 'a' acts as an agent for spreading 'b'.
- Technology: A virus ('b') infected website a is visited by computer 'c'. => Both 'a' and 'c' are infected with 'b'.
- Marketing: Person 'a' impressed with product 'b' tells his friend 'c' about the product. => Both 'a' and 'b' become users of 'c' (Word of mouth marketing)
- Medicine: Person 'a', with an infectious disease 'b', comes in contact with person c. => both 'a' and 'c' are down with 'b'
- Society: Religious preacher 'a' preaching religion 'b' meets and manages to convince person 'c' => Both 'a' and 'c' believe in 'b' (You can replace religion with any set of beliefs)
You can extend this logic to many other sets. Some more examples:
- Matchstick, fire, candle
- Teacher, Knowledge, Student
- Happy person, Happiness, Happy person's friends (Not true for everyone, but perhaps we all have met at least one person, whose zest for life is contagious)
See question on Quora