Jeff McMahan has some very interesting arguments about what we are. Simply put, he argues that we are not our cells. Our cells, our organisms are simply part of ourselves. Based on that I think that we are entities made out of e.g. our cells, immune systems and the bacteria which our organisms carry. Sure, we can make a distinction between 'us' and 'them'; it's just that 'us' actually includes 'them', and that it's just a distinction made by ourselves because we like simple models and hate the feeling of being not autonomous. Humans like to compartmentalize: we see that all the time especially when dealing which psychiatric diseases.

I don’t think I’m my cells or the bacteria living in my gut. Well, that’s not entirely true, I clearly perceive the cells in my hand and the bacteria in my gut as part of myself in some way but not the same way as other things.

Maybe this question is helpful: What do I care about? Clearly not my cells or microorganisms. I would be more than happy to dispose of my hip joint and replace it with titanium should that ever be necessary. I essentially think the same about every other part of my body.

I also don’t care about my DNA at all. If you cloned me after my death I wouldn’t suddenly spring to life, I wouldn’t be reborn as a baby, my clone would be a different person.

What do I care about, then? That, which says this sentence in my brain right now as I type it, whatever it is. It’s probably rooted in structures in my brain somewhere. Which doesn’t mean that I care about the cells in my brain, not at all. If there is a way to preserve that structure and run it I would be just fine. That’s what I care about.

That, to me, is the most important, probably the only really important part of me. The body matters but only in the sense of being a sort of tool.

You should care about your DNA! It isn't just a blueprint, but a computational device that stores an insane amount of state information. In fact, the internal state of your cells as an aggregate (genome/proteome, concentration/expression levels, modifications, etc) hold far more state info than mere synapses.

I understand that we all view our brains as a record of our cognisant experiences. However, there is far more at play at the molecular level than non-biologists usually consider. Brains are not magical "higher level" abstract devices. They work in concert with the existing computational machinery of the cell and the genome.

If you don't buy that argument, at least be mindful that if you don't take adequate care, you will increase your probability of getting a cancer that your immune system cannot fight. Your cells are critical. Your gut flora is critical. You are an ecosystem, not an individual. Your mind is only the deterministic imprinting that the physical world makes on you as you make your way through life.

(I'm a computational biochemist and soon to be systems biology student.)

Ok, then let me rephrase that, I care about whatever it is that makes me think this sentence. I don’t necessarily care about its specific implementation, whatever it is.

This doesn’t mean that I think I shouldn’t take care of my body – I know that it is instrumental to me – I just wouldn’t care much if it were destroyed should I be able to continue be whatever makes me write this sentence.

Indeed, humans like to "compartmentalize"; the problem is that by definition you have to compartmentalize to define concepts. Without that, everybody would be talking horribly vaguely.

I find it pretty cool that snippets of information passed down from my ancestors encoded in my DNA can be synthesised into physical objects, find their way into somebody else's body and permanently write themselves into their DNA. We're all infectious!

Some of those bits of information might even turn out to be useful, millions of years down the line. In fact, they probably are, which is why this mechanism has stayed in our genome despite all the unpleasant side-effects. We share genetic information outside of reproduction, just like bacteria do!