Conclusion
Pregnancy (increased estrogen levels), uncontrolled diabetes, impaired immune system, and antibiotics are common causes of a yeast infection. If a yeast infection is observed frequently, you must consult the doctor with immediate effect.
References:
[1] https://www.nichd.nih.gov/health/topics/vaginitis/conditioninfo/Pages/risk.aspx#f3
[2] https://www.cdc.gov/std/bv/stdfact-bacterial-vaginosis.htm
[3] https://www.cdc.gov/std/trichomonas/STDFact-Trichomoniasis.htm
[4] https://www.cdc.gov/std/tg2015/vaginal-discharge.htm
[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3966341/