Yellow fever and dengue are the pair of diseases that are viral and transmitted by mosquitoes.

Yellow Fever:
- Yellow fever is a viral disease caused by the yellow fever virus.
- It is primarily transmitted by the bite of infected female mosquitoes, especially the Aedes aegypti species.
- The virus is endemic in tropical regions of Africa and Central and South America.
- Symptoms of yellow fever include fever, headache, muscle pain, jaundice, and bleeding.
- In severe cases, it can lead to organ failure and death.
- There is no specific treatment for yellow fever, but a vaccine is available for prevention.

Dengue:
- Dengue is a viral disease caused by the dengue virus.
- It is transmitted by the bite of infected female Aedes mosquitoes, primarily Aedes aegypti.
- Dengue is common in tropical and subtropical regions around the world.
- Symptoms of dengue include high fever, severe headache, joint and muscle pain, rash, and mild bleeding.
- In severe cases, it can lead to dengue hemorrhagic fever or dengue shock syndrome, which can be life-threatening.
- There is no specific treatment for dengue, but supportive care can help manage symptoms.

Viral Diseases Transmitted by Mosquitoes:
- Mosquitoes are known to transmit several viral diseases, including yellow fever, dengue fever, Zika virus, chikungunya, and West Nile virus.
- These viruses are primarily transmitted when a mosquito bites an infected person and then bites another person, transmitting the virus through its saliva.
- Mosquito-borne viral diseases are more common in tropical and subtropical regions where mosquito populations are abundant.
- The Aedes mosquitoes, particularly Aedes aegypti, are the main vectors for these viral diseases.

Conclusion:
- Among the given options, the pair of diseases that are both viral and transmitted by mosquitoes is yellow fever and dengue. Both diseases are caused by different viruses and are primarily transmitted by the bite of infected Aedes mosquitoes.