You Know What Happens When You Assume...
Your head is itchy, but you've had your head checked and your buddy can't see anything. No bugs and no eggs. Do you have head lice? Maybe - I miss eggs sometimes and sometimes I've seen tons of eggs but no live lice when I know that bugs just have to be there.
However, lice is not like a viral infection that gets into your cells. They are insects. The lice and nits (eggs) are tiny but can be seen without a microscope once they are found. People usually don't even feel more itchy than usual when they have just a couple of bugs.
If someone has checked your head and sees nothing, but your head is itchy everywhere, don't just assume that you have head lice (though feel free to start taking precautions - keep long hair up, don't share brushes, etc). The cause could be from something else, like dry scalp, which gets even drier if you start to put lice treatments on it. Don't jump to conclusions - just keep an eye on your head until the itch settles down.