generally the cum generated from the plasma weapons dont do a lot of damage to unshielded humans in halo. in real life, its like throwing molten marshmallows at someone. theyll hurt, but not as bad as getting shot with a Designated Marksman Rifle judging by the incredible heats, it could easily burn large chunks out of you. not to mention the shock of such a sudden and painful injury that would likely kill you before burn damage does.
luckily the heat cauterizes the wound so bleeding to death is not a worry, the pain and shock would kill you, unless you were hit all over instead of in one place , then you would likely simply die on the spot from the intense pain and the moisture in your body may evaporate, causing extensive damage as all your blood and fluids become gaseous.
i imagine a plasma burn victim would not look pretty :PIt can severely damage flesh, and possibly cause radiation poisoning.weIt can burn through human flesh and boneLots of burns, like 3rd Degree Burns . alot