If maniy (semen) is emitted whilst one is awake because of illness, ghusl is not required

Allah says (interpretation of the meaning): 

“If you are in a state of Janaabah (i.e. after a sexual discharge), purify yourselves (bathe your whole body)” [al-Maa’idah 5:6]

Junub (the person who is in a state of janaabah) refers to the person who has emitted maniy which gushes forth, accompanied by feelings of pleasure.

“Gushing forth” refers to the aayah where Allah says (interpretation of the meaning): 

“So let man see from what he is created!

He is created from a water gushing forth” [al-Taariq 86:5-6]

If the maniy is emitted whilst a person is awake, with no feelings of pleasure,  then he does not have to do ghusl. With regard to the hadeeth of the Prophet (peace and blessings of Allah be upon him), “Water is for water” (narrated by Muslim, al-Hayd, 518), it is to be interpreted as referring to the regular case, when maniy is emitted accompanied by feelings of pleasure, and results in tiredness and exhaustion. But with regard to that which is not accompanied by feelings of pleasure, it is does not result in tiredness and exhaustion. 

Hence they said that this water (maniy) has three characteristics: 

1.It comes out gushing.

2.Its smell when it has dried is like the smell of eggs, and if it is not dry its smell is like that of mud and pollen.

3.Tiredness of the body after it is emitted. 

See al-Sharh al-Mumti’ by Ibn ‘Uthaymeen, part 1, p. 277-278. 

The Standing Committee was asked about the obligation of doing ghusl after experiencing a wet dream. They replied: 

It is clear that ghusl is obligatory when maniy comes out gushing, accompanied by feelings of pleasure, when one is awake, and if it happens, regardless of how it comes out, when one is asleep, because Imaam Ahmad narrated from ‘Ali ibn Abi Taalib (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “If water gushes, then do ghusl, and if it does not gush, then do not do ghusl.” 

Fataawa Islamiyyah, vol. 1, p. 216

It was also narrated by Abu Dawood and al-Nasaa’i that ‘Ali (may Allah be pleased with him) said: “My madhiy (prostatic fluid) flowed excessively and I used to keep taking baths until (the skin of) my back cracked. I mentioned that to the Prophet (peace and blessings of Allah be upon him) – or that was mentioned to him – and the Messenger of Allah (peace and blessings of Allah be upon him) said: ‘Do not do that. If you see madhiy, then wash your penis and do wudoo’ as you do for prayer, but if water gushes out [i.e., maniy or semen], then do ghusl.” (Narrated by Abu Dawood, al-Tahaarah, 178; al-Nasaa’i, al-Tahaarah, 193. Classed as saheeh by al-Albaani in Saheeh Sunan Abi Dawood, no. 187). 

Ibn Al-Manzoor said: This indicates that if anything other than that is emitted when one is awake, or it flows because of sickness, then he does not have to do ghusl. 

Ibn ‘Aabideen said: If it – i.e., maniy – is emitted because of being beaten or because of carrying a heavy load on one's back, then in our view he does not have to do ghusl. 

Al-Dardeer said: If it is emitted without feelings of pleasure, rather it flows or comes because of being beaten or being stung by a scorpion, then he does not have to do ghusl. 

The Hanbalis and the Shaafa’is, according to the more correct of the two views, stated that if a man’s back is broken and maniy comes out of him, and it does not come from the penis, then he does not have to do ghusl. The Hanbalis stated clearly that the ruling in this case is like regular kinds of najaasah.

See al-Mawsoo’ah al-Fiqhiyyah, vol. 31, p. 197.

And Allah knows best.