Why does a mark come up on every picture I take?

I've got a Nikon D70s with a Sigma Macro lens. I went to use my camera yesterday but every picture I take there are marks which look like hairs on every picture! I have taken the lens off etc and wiped away any hairs which may be there but nothing has changed. What is this? I need my camera as I do a photography course but the pictures look so terrible with this massive hair-like line across most of it!

We need to see an example to diagnose the problem properly.

Hmm, fungus doesn't normally show up on your images, neither does a hair on the lens.
Maybe a hair on the sensor or the mirror? Do you leave the camera around with no lens or cover on it?
Hopefully it's a hair and nothing more serious.
As the other person said, an example picture will make it easier.

From what you've described, I'm inclined to think that the problem may be something on the sensor. Another possibility could be that it's on the rear lens element rather than the front.

I'll third the opinion that seeing photos would help troubleshooting.

Take the lens off. Give it a good wipe and then use something like a rocket air blower to get rid of any dust inside.

If it's the sensor that's dirty, i'd highly recommend taking it or sending it somewhere to be cleaned professionally, it's not a job you want to attempt and mess up.

If you have cleaned the lens then it is probably a hair on the sensor.

This usually can be removed fairly simply with a hand powered air blower (not compressed air) following any sensor cleaning instructions in the camera manual.

An experienced DSLR photographer should be able to do this simply or a camera repair shop.

http://aviewfinderdarkly.com.au/2007/04/04/how-to-avoid-dust-spots-on-digital-photographs/

If you ahve cleaned the lens, then the next likely culprit is the little mirror inside. For 20$ you can get a cemera cleaning kit form any electronics store. In it will be alittleair blower… Littls squeezy thing with soft bristles at the end. Use it to clean (gently!) the little mirror you see inside with the shutter open.

Otehrwise if that doesn't work, you may have dust on your sensor, which needs a pro to clean it. But usually that leaves really big fuzzy things… Small "hairs" are fibers from your clothes that have migrated inside.

If you are uncomfortable doing this, any camera shop will clean it for you for under 25$.It really only takes five mintues to do…

Hope that helps!