In 2009, The World Health Organization published guidelines which state that all direct patient care and health care workers are prohibited to apply artificial nails. The CDC also published a similar guideline.
Previous studies found that artificial nails allow an ideal environment for fungus and bacteria to grow inside the nail. Especially, older artificial nails can lift at the edge which results in providing an optimal space for the microorganism to thrive.
Surprisingly, the other study found that there is a difference in the number of bacteria between freshly polished nails/natural nails and those that had polish on for more than 4 days (with cracking or chipping). When you have a crack or scar on your nail, you get to have more bacteria present on nails with older polish.
Although the use of nail polish remains controversial, it still makes sense to keep nails freshly polished and sterilize your tools for your healthy nails. Take care of your nails and make sure you are not exposing the nails to the environment where fungi and bacteria are prone to thrive.