In Jain Dharma this question is understood through ahiṁsā + practical maryādā (rules).

1. Basic principle – ahiṁsā towards even bacteria

- Yes, curd (dahi) is full of one-sensed beings (micro‑organisms). - Therefore, Jain shāstras and ācāryas are always cautious about curd and other fermented foods.

1. Why it is still sometimes allowed for śrāvaks (lay Jains)

Traditionally, for householders, five milk products were permitted with care: milk, curd, ghee, butter, whey – when: - milk is first well boiled and filtered, - curd is set fresh the same day, - and eaten before sunset, not kept overnight.

The idea is: - Fermentation has begun, but multiplication of jīvas is still comparatively less in very fresh curd. - A householder’s vows (aṇuvrat) are lighter than a monk’s mahāvrat, so some practical leeway is given, while still trying to minimize (not totally eliminate) this himsā.

1. Monks vs laypeople

- Monks/Nuns (Digambar & Śvetāmbara): in practice, avoid curd and fermented items completely. - Many serious lay Jains also avoid curd regularly, or at least during: - Paryushan / Das‑lakshan - Chaturmās - Upvās, Ayambil, and other special vows.

1. Bacteria issue specifically

- Jainism accepts that bacteria are jīvas. - But daily life already involves killing many micro‑organisms (in air, water, walking, etc.). - Shāstra allows some “inevitable, minimized” micro‑himsā for householders (like boiling water, simple cooking, filtered milk), while asking us to avoid extra / avoidable himsā, such as: - strongly fermented, old, or sour curd, - cheese, yeast foods, alcohol, idli/dosa batter, etc.

1. Different practices today

- Some Jain families: eat only very fresh curd, daytime only. - Some: avoid curd completely, only take milk or buttermilk, or go fully vegan because of modern dairy cruelty. - During Paryushan / strict niyam, usually no curd at all.

Simple way to remember

• Ideal Jain way: no fermentation, no curd – especially for sādhus and strict sādhaks.
• Traditional minimum for ordinary śrāvaks:

- If taken, only same‑day, fresh, from boiled milk, before sunset, and avoid on special vrata days.

• The higher level of ahiṁsā is to reduce or leave curd and all fermented foods.

You can read more on curd in Jainism and related food rules here: Curd in Jainism Why Jains do eat curd even if it has bacteria