HTLV-1 is a blood-borne retrovirus that causes leukemia in about 3% of those carrying it. In southern Japan, roughly 4% of the populace are afflicted with this virus, so are some isolated groups living today in Columbia and Chile. Does this correlation prove that South America received settlers from Japan in the distant past? Such a biological linkage would augment pottery evidence from Peru and, especially, Ecuador where Jomon-style pottery 4,000-5,000 years old has been found on the coast. However, the HTLV-1 virus also could have been introduced to South America by more recent visitors. Is there any way to fix the timing of HTLV-1's introduction to South America?
Actually, there is. The DNA in viruses is not as durable as pottery shards, but it does hang around for a while, as seen is recent efforts to ex-tract DNA from from frozen mammoths for possible "revival" of the species.
A team of Japanese and Chilean scientists has been searching for DNA surviving in 104 mummies deposited in South America's arid Atacama Desert 1,200-1,500 years ago. Two of the mummies still retained DNA; and one of them included shards of DNA from HTLV-1. This certainly doesn't prove trans-Pacific diffusion, but it helps.
(Holden, Constance; "Backtracking a Mummy Virus," Science, 286:2071, 1999.)