I know there are some people that have a disease but do not have the symptoms of it, called carriers. But I don't quite understand how that works. Help?
Thanks,
~~Ali. 
They have a gene and they carry two alleles in it. one is a healthy allele which is dominant so the person is healthy. the other allele carries the disease and is recessive meaning it is there but not doing anything as the healthy allele is dominant. Hope i t cleared things up a bit.
well it can depend on the disease and even then it can still be a mystery.
Basically the person who is the carrier may be slightly genetically different to other people meaning that while the disease can survive in them it can't take hold.
or maybe their immune system has adapted in such a way from another illness as to not cure or prevent the disease but to arrest it from damaging them.
It could also be by design of the disease, wipe out all hosts before you can spread and it potently wipes it self out, so maybe the disease actually chooses not to infect the host.
there are many reasons and answer for the why's, yyour best bet would be to do some research both online and off.
I'd suggest starting with typhoid mary, possibly the worlds most famous carrier.