Is the fast invalidated if pus comes out of the body?
The fasting person is not affected if blood comes out of his wounds and that does not invalidate his fast. Rather the difference of scholarly opinion has to do with wet cupping in particular. The more correct opinion is that it does invalidate the fast because of the text which indicates that, which is the words of the Prophet (blessings and peace of Allah be upon him): “The cupper and the one for whom cupping is done both break their fast.”
Narrated by Abu Dawood, 2367; Ibn Maajah, 1679. Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked: If a person is fasting and he bleeds, does he have to break his fast or should he complete it?
He replied: Bleeding does not affect him, except in the case of cupping, if he is treated with cupping. The correct view is that the fast is broken by cupping. There is a difference of scholarly opinion concerning the matter, but the correct view is that it does break the fast, because the Prophet (blessings and peace of Allah be upon him) said: “The cupper and the one for whom cupping is done both break their fast.” But if he has a nosebleed or is injured in his foot or hand when he is fasting, his fast remains valid and that does not affect him.
End quote from the Shaykh’s website on the following link:
Some of the scholars differentiate with regard to bleeding in general between that which occurs as the result of a person’s actions and choice and results in a lot of blood coming out, as in the case of donating blood, for example, which does break the fast, by analogy with cupping, and that in which he has no choice, such as injuries and the like, which does not break the fast even if there is a lot of blood. This has been discussed previously in the answer to question no. 37918.
With regard to pus and the like coming out of wounds, this does not affect the one who is fasting. It says in ad-Diya’ al-Laami‘ min al-Khutab al-Jawaami‘ (5/465), by Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him): The fast is not broken by lancing a wound to drain the pus from it, even if blood comes out. End quote.
And Allah knows best.