Conclusion
The flu can occur at different times, depending on where you reside. It is always around, but more active in the winter and less active in the summer. Studies show that sunlight may reduce the effects of the virus, but more research needs to be done. The virus can be transferred through tiny droplets from a contaminated person.
References;
[1] https://medlineplus.gov/immunesystemanddisorders.html
[2] https://www.cdc.gov/flu/swineflu/index.htm
[3] https://www.healthline.com/health/fatigue
[4] https://vicks.com/en-us/symptom/nasal-congestion
[5] https://www.nhs.uk/conditions/headaches/
[6] https://kidshealth.org/en/parents/fever.html