The other poster mentioned that several kinds of common mutations appear to prevent certain kinds of diseases. What's actually happening is that the people die off from the mutations before they can catch the diseaaases. I'd just as soon not be saved in that manner myself.
The other poster was myself, and your statement is false.
I'll use sickle-cell as an example. The simplified version is
1) if you're heterozygous for both normal and S-hemoglobin, you are perfectly normal, but you are*much* less likely to die of malaria. There is nothing preventing you from passing your genes along.
2) If you are homozygous for normal hemoglobin, you die early of malaria, and your genes aren't passed on.
3) If you are homozygous for S-hemoglobin, you die of sickle-cell anemia, and your genes aren't passed on either.
From the The American Sickle Cell Association:
Studies show that in areas where malaria was a problem, children who inherited one sickle hemoglobin gene and who, therefore, carried the sickle cell trait - had a survival advantage. Unlike the children who had normal hemoglobin genes, they survived the malaria epidemics they grew up, had their own children, and passed on the gene- for sickle hemoglobin.
That is ridiculous.
Until recently, those that had two copies of the 'sickle cell' allele died. Those that had one copy of the 'sickle cell' allele generally survived malaria. Those that had no 'sickle cell' allele generally died from malaria. Having one copy is an advantage, having two is not. For a person to have two copies, both parents would have to have the allele. Since gametes are the random selection of one of two alleles at any given locus, there is only about a .25 probability of a child ending up with two 'sickle cell' genes.