Ring worm treatment

Pearl, you sound like an awesome big sister! Your little sister is really lucky to have someone like you watching out for her and helping her to learn about her own health. Taking her to the doctor AND making sure you understand the truth yourself is a winner combination. It sounds to me that you really are on the road to becoming a great Pediatrician
Infections by worms have been recognized since antiquity. Treating some of these infections was among the few notable successes of early doctors. Physicians adopted as their professional symbol a staff encircled by two worms. This symbol, the caduceus, is used to this day.
One of the more obvious worm infections was one which we today call cutaneous larva migrans. As the worms travel through the body just under the skin, scaly red lines mark their paths. These lines are actually inflammatory reactions to the worms.
A very similar skin finding, with raised scaly rings instead of straight lines, was also noted in antiquity.
This infection came to be called ringworm, since the worms apparently traveled in laps around a short circle, rather than in a wandering line. Ringworm typically lasted several months and then, thankfully, resolved on its own. The infection was known to be mildly contagious, but the worm itself was never seen. It was a great mystery.
Some people thought that the ringworm skin lesions looked rather like the holes made in garments by moths -- flat in the center, with a raised border. During the Middle Ages physicians began calling this infection tinea, the name of a common family of moths. Physicians added a descriptive word to the name to specify where on the body the infection was located: ringworm infections on the head were called tinea capitis, on the body tinea corporis, on the feet tinea pedis. They also had specific names for the same infection found in the groin, armpit, facial hair, or nails.
It was not until 1837 that the true cause of ringworm was found. A Polish physician looking at scrapings from the skin of people with ringworm identified a fungus (that he then massaged into his own arm!). Over the next several years, a number of investigators proved that what we call ringworm is always a fungal infection. This was the first time that any microscopic organism was ever proven to cause human disease. It has no connection at all with worms, other than the similar appearance of the skin lesions.
Fungi are tiny plants that survive by eating plant or animal material. The ringworm fungi feed on keratin -- the material found in the outer layer of skin, hair, and nails. These fungi thrive best on skin that is moist, hot, and hidden from the light. When this infection is found on the feet, it is commonly called athlete's foot; when it is found in the groin it is commonly called jock itch; and when it is found on the body it is still called ringworm. Up to 20 percent of the population has one of these infections at any given moment.
Ringworm is very mildly contagious. It can be caught from domestic animals (especially dogs and cats) as well as most farm animals. The infection can be caught from the animal directly, or from anything the animal rubs against (yes, Pearl, your sister could have gotten it from playing on the ground at the BBQ). Ringworm can also be caught from other humans, both by direct contact and by prolonged contact with flakes of shed skin (from sharing clothes or from house dust, for instance). Wrestlers commonly spread it back and forth with their sweaty contact (tinea gladiatorum!).
To catch ringworm, you have to be exposed to it and you have to be susceptible. Some people are much more susceptible than others. Those with eczema or other skin problems get ringworm more easily because the protective barrier of the skin's outer layer is less intact. Children are more susceptible before puberty. Boys get it more easily than girls. Some people are genetically predisposed, and can get it easily throughout life (like me!).
The treatment for ringworm is one of the many effective topical antifungal creams, such as miconazole (Tinactin) or clotrimazole (Lotrimin). Several of these antifungal creams are now available without a prescription. Treatment may require several weeks. Only by treating for at least one week after the resolution of symptoms (and for a total of at least 4 weeks) can one guarantee eradication. (Pets can be treated with the same medicines, but this is difficult, since they often don't get a rash with their infections. Contact your pet's veterinarian to get up-to-date information on the best treatment for animals). As soon as treatment has begun it's fine for her to play with others, but it's best not to share clothing or to let other children rub the patch of ringworm.
Rarely, when ringworm of the body is resistant to topical therapy, we use oral antifungal medications for about one month.
When ringworm is found in the nails or on the scalp the infection is much more difficult to eliminate. Prolonged treatment with a prescription oral anti-fungal medicine (as well as other topical medicines) is usually necessary. Scalp ringworm is a major cause of hair loss, and should be treated aggressively.
Pearl, I expect that your sister's patch of ringworm on her leg will clear up with the Tinactin. In the meantime, thanks for watching out for her!