Does Impurity Transfer?
Question

I am confused about the issue of purity (Taharah) and how a person can remain in a state of purity. I read on your website that impurity (Najasah) can be transferred through things that are wet, but not through things or surfaces that are damp. What about other semi-liquids such as honey or creams – can impurity be transferred through them? I take care of a sick person who is not able to walk, and as part of my duties towards him, I clean him after he defecates. What is the ruling in this case? Can the impurity be transferred from a wet surface to something else? Does that also apply to impure substances of all types, whether they are visible or not visible? Can impurity be transferred to a sweaty hand? What if I step into impure water (sewer water, for example), then bring my feet out and leave them without washing until they dry, then put on socks, then my feet and the socks get wet because of sweating – do the socks become impure (Najis)? I am confused about all these details, and I hope that you can explain.
Praise be to Allah.Can dry impurity transfer?
We have never previously on our website differentiated between wet things and damp things with regard to the ruling on impurity. Rather what is wet and what is damp come under the same ruling in that regard. The difference is between things that are wet (or damp) and that which is dry.
Dry impurity cannot be transferred if it comes into contact with another dry surface. This is something that is quite clear, because the dry thing does not develop any of the characteristics of the impure substance (colour, taste or smell) by merely coming into contact with something else that is dry like it.
As-Suyuti (may Allah have mercy on him) said: “Principle: Al-Qamuli said in Al-Jawahir: If something impure comes into contact with something pure and they are both dry, that does not make it impure." (Al-Ashbah wa’n-Naza’ir,  1/432)
Shaykh Ibn Jibrin (may Allah have mercy on him) said: “Contact of an impure substance that is dry with a body or clothes that are dry does not affect anything, because impurity can only be transferred when it is wet." (Fatawa Islamiyyah, 1/194)
But if it is wet, then it is most likely that it could be transferred, whether it comes into contact with something that is wet or that is dry.
Are honey, cream and other semi-liquids affected by impure substances?
Honey, cream and other semi-liquids are affected by impure substances if they come into contact and mix with them, therefore they become impure and the impurity may also be transferred to other things through them, especially if the amount into which the impurity fell is small, which would lead to the impure substance spreading through it, and it is not so large an amount that the impact of the impure substance is limited only to the part of the semi-liquid that surrounds it.
It was narrated from Ibn `Abbas (may Allah be pleased with him), from Maymunah (may Allah be pleased with her), that the Messenger of Allah (blessings and peace of Allah be upon him) was asked about a mouse that fell into some ghee. He narrated from Maymunah that a mouse fell into some ghee and died. The Prophet (blessings and peace of Allah be upon him) was asked about that and he said: “Throw it out, along with what surrounds it, and eat [the rest].” (Narrated by Al-Bukhari, 5538)
The Prophet’s words “along with what surrounds it” indicate that what surrounds the impure dead animal has been affected by its impurity. There is no scholarly difference of opinion concerning that; rather the difference of opinion has to do with the semi-liquid other than that.
Whatever the case, what matters here is that semi-liquids are affected by impure substances, just as water is affected, and more so, because water can ward off impure substances to a greater degree than semi-liquids can.
Transfer of visible and invisible impurity
There is no differentiation between visible and invisible impurity, such as the impurity of urine, for example, so long as it is known that it has reached the place, or any of its characteristics appear in that place. The place becomes impure when one of the three effects of impurity appear in it, namely its colour, its taste or its smell. If we assume that urine fell onto a garment, and the colour of the garment is such that no trace of that urine can be seen, this does not mean that the garment did not become impure.
But if the impurity is a small amount, such that it cannot be seen by the naked eye, such as small droplets of urine and the like, then some of the scholars say that it is overlooked.
Ash-Shirazi (may Allah have mercy on him) said: “If the impurity is something that cannot be seen, then there are three views concerning it. Some of our companions said that it (the impurity) does not affect any Fiqhi ruling, because it cannot be avoided, so it is like dust from manure. Some of them say that it comes under the same ruling as all other impurities, because it is definitely impure, so it is like impurity that can be seen…”
An-Nawawi (may Allah have mercy on him) said: 
“The correct view out of all of these is that neither water nor garments become impure [as a result of droplets falling on them]. Al-Muhamili stated definitively that this is the correct view in Al-Muqni‘, and he quoted it in his two books from Abu At-Tayyib ibn Salamah. It was also regarded as correct by Al-Ghazali and the author of Al-‘Uddah, and others, because it is not possible to avoid it and because it may cause hardship if you try to avoid it. Allah, may He be Exalted, says (interpretation of the meaning): {and [Allah] has not placed upon you in the religion any difficulty } [Al-Hajj 22:78]. And Allah knows best." (Sharh Al-Muhadhdhab,  1/178)
Al-Mirdawi (may Allah have mercy on him) said: 
“With regard to what may be overlooked of impurities, that includes what it says in Ar-Ri`ayah: A small amount of impure water may be overlooked if it is like the amount that may be overlooked of blood and the like, according to the more correct opinion. He favoured the view that the small amount that cannot be seen may be overlooked… Shaykh Taqiy Ad-Din favoured the view that a small amount of all types of impurity may be overlooked, in food and other things, even mouse faeces. He said in Al-Furu`: That means that he chose the view of the author of An-Nazm. I say: He said in Majma` Al-Bahrayn: I say: It is more appropriate that the small amount may be overlooked in clothing and food, because it is too difficult to avoid it, and no man of reason will doubt that this is a widespread problem, especially in mills, sugar processing facilities and olive presses. It is more difficult to guard against than water from which mice have drunk, and the blood and vomit of flies. Many of our companions adopted the view that it is pure." (Al-Insaf,  1/334).
And it was said: “A small amount of impurity is not to be overlooked , even if it is not visible. (See: Kashshaf Al-Qina‘, 1/190)
Importance of hastening to remove impurity
If an impure substance gets onto a person, the Sunnah is to hasten to remove it, and not leave it until it dries, because of the Hadith of Anas ibn Malik (may Allah be pleased with him), who said: A Bedouin came and urinated in the mosque, and the people rebuked him, but the Prophet (blessings and peace of Allah be upon him) told them not to do that. When he had finished urinating, the Prophet (blessings and peace of Allah be upon him) instructed that a bucket of water should be thrown over it. (Narrated by Al-Bukhari, 221)
Ibn Hajar (may Allah have mercy on him) said: “This indicates that one should hasten to remove bad things when there is no reason to delay doing so, because he instructed them to pour water over it as soon as he had finished." (Fath Al-Bari)
Does impurity transfer through sweat?
If the feet on which there is impurity become sweaty, the impurity will inevitably be transferred to the socks, because wet impurity – as noted above – is transferred to other things, even if the place to which it is transferred is dry.
And Allah knows best.