The answer to 'why is my patio disease' is that your patio may be suffering from a variety of issues, such as mold, mildew, or other types of fungal growth. These problems can be caused by poor drainage, excessive moisture, or even improper construction. Symptoms may include discoloration, cracking, or a slippery surface.
To address the issue, you'll need to first identify the underlying cause. This may require inspecting the patio, testing the soil, or even consulting a professional landscaper. Once the problem is identified, you can take steps to resolve it, such as improving drainage, replacing damaged materials, or treating the affected areas with a fungicide.