As you read this billions of microscopic machines are tearing through my body, breaking down my cells and converting me into yet more replicators, multiplying by a thousandfold every 45 minutes. There was only one at first, but now they outnumber the cells in my body. They have already devoured many of my mucous membranes, leaving open sores. My lungs are starting to fail, making it hard to breathe and my entire body is weakened. Similar bioattacks have killed in mere hours, but I am fighting them off. There is nothing medical science can do except sit by and hope I recover.
And that is the 'flu. It always seems far more awesome when you remember that viruses are in fact nature's Ultron.