Painful, red and itchy skin around your nails is a pretty big clue that something's not right. Just like other parts of your body, your fingernails and toenails are prone to infection, usually occurring in adults and caused by fungus (such as yeast), bacteria (such as Staphylococcus) and viral warts. Nail infections don't necessarily indicate larger, systemic health problems but they do need to be treated by a doctor, especially if you have a medical condition that weakens your immune system.
Fungus is the most common perpetrator, infecting about 12 percent of Americans [source: American Academy of Dermatology]. It can cause nails to become thick and crumbly and change color, taking on a blue-green hue. Fungus is notoriously difficult to treat, so see a doctor for medicine and expect to see results only after your nails have gone through a complete growth cycle (a few months).
Bacteria and viruses also both cause unsightly changes to nails. Bacterial infections target the skin under and around the nail and can lead to nail loss if not treated. Skin viruses cause warts around and sometimes under the nail, which a doctor can freeze off or chemically treat to remove.
Unkempt artificial nails, unsanitary manicure equipment and vigorous manicuring can all increase the chances of infection. Always be sure to properly -- and gently -- clean your nails, fingernails, toenails and artificial nails, and buy your own manicure tools to reduce the spread of bacteria from person to person.
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Fingernails can grow to extreme lengths. Learn more about how long and fast fingernails can grow in this HowStuffWorks Now article.