Firstly: to learn more about the ruling on the najaasah (impurity) of blood, see the answers to Questions 2570 and 2176. 

Secondly: some of the scholars said that if water (pus) from wounds is a small amount, it does not invalidate wudoo, but if it is a large amount then it does invalidate wudoo’. 

A group of scholars – including al-Shaafa’i and Imaam Ahmad, according to one report narrated from him, and the seven fuqahaa’ – said that anything that comes out from anywhere other than the front and back passage does not invalidate wudoo’, whether it is a small amount or a large amount, apart from urine and faeces. 

They quoted the following as evidence for that: 

1-The basic principle is that it does not break wudoo’; whoever claims that something breaks wudoo’ has to produce evidence.

2-The fact that he is taahir (pure) is indicated by the shar’i evidence, and whatever is indicated by shar’i evidence cannot be changed except by further shar’i evidence.

We do not go beyond that which is indicated by the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him), because we worship Allaah according to His sharee’ah and not according to our whims and desires. We have no right to oblige the slaves of Allaah to do wudoo’ when it is not required, or to excuse them from doing wudoo’ when it is required. 

See al-Sharh al-Mumti’ by Ibn ‘Uthaymeen, part 1, p. 224