I think the question is based on a misapprehension. Kuru and Mad Cow Disease are not caused by cannibalism. This seems to be quite a common belief and is a misunderstanding. The cannibalised individual has to be infected in the first place. Which is relatively rare. What can be said, perhaps, is that cannibalism is possibly the only way the infection can be transmitted from individual to individual as consumption of infected tissues is pretty much the only way it can pass from person to person. However, kuru is not a fundamental risk of cannibalism in and of itself. If there isn't kuru in the population already, cannibalism itself isn't going to cause it to emerge.

I think the myth came from a group of people who engaged in ritualistic cannibalism of their dead. They had kuru in the population already from eating infected ungulates and people were dying of kuru and then being eaten due to the traditional practices, which just passed the problem on. But the cannibalism wasn't the source of the condition, just the means of transfer.

Focusing more on your second sentence and taking it down ro the simplest level: imagine after a haircut someone was tasked with reattaching each cut strand of hair to the strand on your head that it *originally" came from. That would be an easier task.

Solar disinfection is used effectively in remote communities in the developing world. The effect is amplified by leaving the bottles on reflective surfaces. It's not great and it takes hours but it is effective.

See here for an academic source: https://link.springer.com/referenceworkentry/10.1007/978-3-319-70061-8_125-1#:~:text=Solar%20disinfection%2C%20or%20SODIS%2C%20refers,at%20least%206%E2%80%938%20h.

Wiki: https://en.m.wikipedia.org/wiki/Solar_water_disinfection

It is a lot, yes. But they still have a bit of a point. In absolute terms lung cancer isn't hugely common in smokers. About 1 in 10. So you're still somewhat unlikely to get lung cancer as a smoker. Although you're massively more likely to get it compared with someone who has never smoked. So it's a huge relative risk but a somewhat low absolute risk. As an individual even if you smoke the odds are still in your favour wrt lung cancer. Of course, with smoking there is a bunch of other things that can kill you before you get the chance to win the prize of lung cancer. So that has to be considered too.

It's very crudely similar to the reason you can't get an organ transplant from your dog despite sharing the same organs. There are certain molecules (mainly proteins) that exist on the surface of animal cells that allow other species to recognise them as foreign. By and large the same kinds of proteins (we say they are homologous) exist on the surface of your dogs cells and yours but they are sufficiently different that your body will recognise them as "not mine".

Viruses exploit their hosts by binding to proteins on the surface of cells. Often these proteins are sufficiently different between species that a virus cannot recognise "Protein X" in a human, even though it operates by binding to the homologous 'Protein X" in a dog. However, it is certainly possible for viruses to evolve to recognise both dog and human Protein X. This can happen most easily when humans and populations of sick animals live closely together for long periods of time. It's one of the leading hypotheses of the emergence of covid, HIV and ebola (to name a few high profile but far from comprehensive examples).

The viruses don't do this on purpose of course. They're mutating randomly all the time. But if a few virions by chance develop a mutation that makes them less selective wrt the differences between Protein X across species AND they have the opportunity to spread to that new species, they can jump the "species barrier" and become a new infectious agent in a new set of hosts.

Question is too vague. It's obvious that there are appreciable differences in the smell of poop. If you've lived for any length of time you must be aware of the fact that, while always generally unpleasant, the specific smell can change quite a bit. Which makes me wonder if you've fully explained your question.

I feel like you need to provide more context about why you're asking this. Or maybe you're asking whether it's nornal if yours smells different occasionally, in which case the answer is: yes, totally normal.

I can talk about your question on serotonin and dopamine (not so much testosterone).

Serotonin and dopamine are not "happy chemicals". That is a very common misunderstanding and a huge oversimplification. They are neurotransmitters with an enormous number of roles in the body, some of which are indirectly related to regulation of moods and emotions and most of which are not. But they do not, in and of themselves, cause "happiness" or feelings of reward. They're a means by which neurons communicate with eachother but they don't themselves carry information about what the message being communicated should be. Their actions are highly context dependent and depend on myriad other influences (neuroanatomical context, receptor type, modulating influence of other neurotransmitters...it's extremely complex). Trying to understand mood by looking at a neurotransmitter is a bit like trying to understand how a car works by studying the chemistry of the materials it is made from.

I kind of think of neurotransmitters sort of like sending messages in a large office. There might be several ways you can do that (email, written note, formal letter, telephone, face to face). Each method being analogous to a different neurotransmitter. Different methods may be more or less appropriate for different kinds of message but knowing the means of communication tells you nothing about the message itself. This is (very much simplified) the mistake people are making when they talk about serotonin and dopamine being "happy chemicals" and quote scientifically illiterate ideas like "dopamine hit".

We don't actually know how SSRI's work either. A lot of the "serotonin is a happy chemical" stuff actually comes from the observation that SSRIs do work, not the other way around. "SSRIs work therefore serotonin must improve mood". But there is growing evidence that their mechanism of action WRT depression may not be anything to do with serotonin at all. At least not directly. I mean they definitely do inhibit reuptake of serotonin but that is not necessarily how they are having their effect on mood.

This 2022 paper gives a good overview of where we're at with that stuff: https://www.nature.com/articles/s41380-022-01661-0

>The main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations.

While this is a very good answer I would not agree with:

>If you don't have any known predisposition towards cancer, then it's probably not something to worry about.

There are certainly things that massively increase your relative risk of cancer even if you don't have a genetic predisposition. I know you already said that exposure to strong carcinogens may cause you to develop cancer, and this may be talking about specifically things like cosmic rays, but the context of this sentence as a stand alone paragraph in your answer makes it sound like a more broad statement.

Smoking is definitely something that will significantly increase your risk of cancer even without a genetic predisposition to an extent that it's entirely appropriate to worry about it. If you have a genetic predisposition it's worse but it's still worrisome without one. There are others too but this is a big one in terms of common lifestyle factors.

Cuts of meat that requires long, slow cooking usually have a lot of connective tissue which takes longer to soften than muscle tissue. The large structural proteins like collagen in connective tissue are more stable and heat resistant.