The study looked at how HIV-1 mutates and affects its ability to replicate in the body. By analyzing genetic data from untreated HIV-1 patients over time, researchers found that mutations occur at a rate of 1.2 x 10^-5 per site per day. Most mutations that change the virus's genetic code make it less effective at replicating. Half of these harmful mutations have a big impact on the virus's fitness, while most harmless mutations have little effect. The cost of harmful mutations is consistent across different patients, suggesting that it plays a big role in the diversity of HIV-1 strains worldwide.