As Sajid pointed out, thalassemia does give some protection from malaria. Thals tend to get less severe cases and can also get protection against the more serious type of malaria by having been infected by the weaker version of malaria. Because of this, thal carriers survived malaria epidemics at much higher rates than non-carriers and while non-thals died, thal carriers didn't, thereby increasing the percentage of carriers in those areas. In this way being a thal carrier has given a huge benefit to those who live in the areas of the world where malaria is prevalent.
Thick blood is not a good thing as it does increase the risk of stroke. Aspirin is often recommended as a blood thinner but I prefer vitamin E.