Americans spend more than $8 billion annually at nail salons. You'd think that would keep toenails and fingernails in pretty good shape.

But according to the Centers for Disease Control and Prevention, up to 14% of people have fungal nail infections, and many people contend with everything from psoriasis-related nail-lifting to bacteria-related greenish-black blotches or melanoma-related black streaks on their nails.

You need to nail down what your nails are telling you about your health. If you have any of these potential signals of infection or disease, see a dermatologist or internist for prompt diagnosis and treatment.

Other color changes tied to specific risks include: yellow nails can come from contantly wearing polish or be a sign of lung disease, rheumatoid arthritis, or infection; half-pink, half-white nails are associated with kidney disease; dusky-red half-moons could be a sign of lupus, heart disease, alopecia, or arthritis; and white nails may signal diabetes or liver disease.

In addition:

• Oddly curved nails (called clubbing) may signal issues with your lungs, heart, liver, stomach, or intestines.

• Pitted nails may be a sign of alopecia, psoriasis, or atopic dermatitis.

• Brittle nails are a common sign of low thyroid function (hypothyroidism).

When choosing a nail salon, make sure it has the highest hygiene standards. You also want to avoid the toxic trio of toluene, formaldehyde, and dibutyl phthalate found in many nail polish products. Ask your salon if they have products that are free of those chemicals.

And make sure they use a liner in all footbaths and an autoclave sterilization device on all reusable tools.