Fungal infection can happen to anyone. Antifungal medicines will kill the present infection . With fresh sweat accumulation new organisms will grow. So most important is to keep the area dry, sweat free, clean. Take bath twice daily , avoid wearing unwashed clothes. You can apply ketoconazole cream twice daily after bath. Oral medications cant be prescribed without a doctor diagnosing it as tinea . As there are other skin problems which a lay man cant recognise.