[paul.fr]

Neils question about pus raises this one. Sometimes when you remove a plaster, you get terrible smell. i reckon this is the smell of the pus on the plaster. is it, and why does it smell?

[RD]

Natural selection has caused us to be sensitive to smells associated with diseases.
e.g. excrement, pus.
So everything produces a chemicals which could be detected by olfactation but we have evolved sensitivity to smells associated with harm and find them offensive.

[dkv]

Offensive and defensive is a matter of Pleasure or Pain.
A good fart which is a natural process produces no smell and there is no need to declare it offensive... but due to social dependency of certain behaviours
farting is not considered a good practice. A fart can reduce your chances of social acceptebility.
Farting also produces smell which can give a good hint about the stomach's good or bad health.
The offensive understanding is due to Group Norms...
The farting is not only necessary but must to get some idea on the health of stomach. The color of the stool and its thickness is also important... direct physical tests can done by Physicians...  BUT this is done by a professionals only.
These declarations of offensive and attractive have developed due to opportunists strategy towards Pleasure. Individuals fake for pleasure. They use condoms for pleasure. (THERE IS NO NATURAL DESIRE AND HENCE NO NATURAL SELECTION TO REPRODUCE WHILE USING CONDOMS OR MASTURBATING OR SAME_SEX PLAY )
However you will notice that when alone natural instict to smell the fart ... or the anus (in animals or humans) In such cases the desire to compute the true function of farted smell.What makes one smell is anybody's guess:-)) (I am sure they dont know about this theory in general)
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Pus is an important tool to give the carrier an idea about the state of wound.This simple fact is known to most of us ....

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If TSP is irritating then it is not worth it ...
The purpose of any Theory is to generate pleasure.

The underlying truth is we adapt towards what brings us maximum pleasure. We do not try to choose or execute biological strategies which are mechanical to the purpose of biology.(farting , sneezing , itching ,coughing etc)

[paul.fr]

dkv, thanks for the reply. i'm not sure that most of it actually relates to the question though.

[dkv]

It smells to give the information about the wounded organ or the part of the body.
It gives information about the health.