Human mutations may increase resistance to some infectious diseases. Humans which carry the mutant gene, the "sickle cell allele" , may be more resistant to death caused by - and humans which carry the cystic fibrosis allele may be more resistant to death caused by

1 - Malaria

2- Typhoid fever and Tuberculosis

Explanation:

In case of malaria, the causal microorganism is Plasmodium falciparum, which target red blood cells of host. In humans who have defective sickle cell allele, have abnomral shaped red blood cells. So microorganism does not able to attach to abnormal shaped RBCs thus humans having sickle cell allele are more resistant to death caused by malaria.

In cystic fibrosis, mutation on that gene which encodes a protein known as cystic fibrosis transmembrane conductance regulator (CFTR).

Normally Salmonella typhi (bacteria responsible for typhoid fever) enters into intestinal cells by attaching with CFTR receptor. Carriers of cystic fibrosis mutant gene, have abnormal CFTR thus reduced rate of entry of bacteria into carriers.

Normally Mycobacterium tuberculosis (bacteria responsible for tuberculosis) adds sulfate in its cell walls. In carriers of cystic fibrosis, arylsulfatase B activity is reduced which incorporates sulfate. Thus reduced arylsulfatase activity makes bacteria unable to add sulfate in its cell walls thus decreases their virulence to carriers of cystic fibrosis.