Here is a Jain perspective in simple terms:

• Core principle: Ahimsa (non-violence) towards all jivas (living beings), including microbes, is central. Killing or harming life, even tiny organisms, is a form of violence.
• Why boiling is discussed: When you boil water to kill microbes, you are causing the death of those tiny jivas. From a strict Jain view, this is a form of violence.
• How it is handled in practice: Jain tradition recognizes the need to protect health and life, especially during fasting or when health could be endangered. In such situations, many practitioners accept a limited, necessary harm as a smaller, unavoidable act of violence in order to preserve greater well-being and the ability to continue one’s spiritual vows.
• The key attitude: Do the minimum harm possible, with awareness, remorse, and mindfulness. If you choose to boil water for health reasons, do so with a sense of responsibility and thankfulness for the life that is being affected, and try to minimize harm in other ways (for example, using clean sources, avoiding needless waste, and following medical guidance).
• Differences between Digambar and Shwetambar: Both traditions uphold non-violence and compassion for all life, but they may differ in how strictly they apply specific practices during fasting. In general, both would allow necessary measures to protect health, while emphasizing remorse and intention to minimize harm.

If you want, I can outline brief guidelines a Jain lay practitioner might follow when deciding about boiling water during a fast (e.g., weighing health risk, seeking medical guidance, pursuing gentle purification methods, and reflecting on ahimsa).