We got two issues here, boys and girls:
1) Bad hygiene
OK, we all agree this is sad, whether through carelessness or ignorance. Not everyone needs to shower daily, but certainly everyone should shower/bathe/whatever frequently enough to not offend those around them. For some, that might be a minimum of every couple of days (if they catch the pits in the morning and any other especially stinky bits). For most, every day or every other day. For some, more often than that.
2) Bad biology
Some folks have overactive sweat glands. Some folks have some weird biochemistry that just makes them reek no matter what they do (I had a boyfriend once who, for about 4 months, stank to high heaven even right after a shower) If they’re lucky, it’s temporary.
But some medical conditions can cause distinctive and nasty odors. Kidney disease, for instance, can manifest some impressive stench. So can cancer. Certain skin disorders can result in body odor strong enough to stun a rhino (John Merrick used to bathe at least twice a day, yet his personal atmospheric effects were frequently commented upon by those around him - and that in Victorian England, where bathing once a week or far less than that was common and deodorant unknown). A cut, rash, boil, or infection in the skin under the arms can make it impossible for a person to wear deodorant, which may cause problems for even those who normally smell quite acceptable. (I’ve occaissionaly suffered from underarm skin infections, which makes me extremely paranoid about odor, which just makes me sweat more… I wind up washing the pits at least twice a day and trying very hard not to engage in physical exertion)
And, for those unfortunate enough to have a olfactory disfunction, they may simply be unaware of their objectionable miasma.
I think, sometimes, when a person appears to have adequate grooming one must simply stand upwind in order to maintain civility. Unless you have a close enough relationship to be able to discuss the problem but man, you have to be REAL careful about that!