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- This is a sub-article to Islamic hygienical jurisprudence.
In Islamic law, najis (Arabic: نجس) are things or persons regarded as ritually unclean. According to Shi'a Islam, there are two kinds of najis: the essential najis which cannot be cleaned and the unessential najis which become najis while in contact with another najis.
Contact with najis things brings a Muslim into a state of ritual impurity (najāsa (Arabic: نجاسة); its opposite would be taharah, ritual purity), which requires undergoing purification before performing religious duties, such as regular prayers.
Islamic law 
According to the Shafi'i school of Sunni Islamic jurisprudence, as systematised by al-Nawawi in his book Minhadj, the following things are najis: wine and other spirituous drinks, dogs, swine, dead animals that were not ritually slaughtered, blood, excrements, and milk of animals whose meat Muslims are not allowed to eat. Spirituous drinks are not impure according to the Hanafi school, while living swine and dogs are not impure according to the Malikis.
Additionally, any meat of animal which is killed in a manner other than that prescribed by Islam is najis.
Najis things cannot be purified, in contrast to things which are defiled only (mutanajis), with the exception of wine, which becomes pure when made into vinegar, and of hides, which are purified by tanning.