Step 1: Understanding the viruses.
Small pox is caused by the variola virus, not the morbilli virus, which causes measles. Other pairs in the options are correct.
Step 2: Analyzing the options.
(A) Yellow fever -- Flavi virus: Correct. Yellow fever is caused by the Flavi virus.
(B) Swine flu -- H1N1 virus: Correct. Swine flu is caused by the H1N1 strain of influenza virus.
(C) Small pox -- Morbilli virus: Incorrect. Small pox is caused by the variola virus, not the morbilli virus.
(D) AIDS -- Retro virus: Correct. AIDS is caused by the retrovirus HIV.
Step 3: Conclusion.
The incorrect pair is (C) Small pox -- Morbilli virus, as small pox is caused by the variola virus.