Some dangerous microbes lurk out there, and in this case; the video is talking about just some of them, retroviruses. These are extremely small microbes, but their size belies their power. They are made up of only a few ingredients that allow it to penetrate and infect cells.
Viruses hijack the machinery of infected host cells to replicate itself. That replication continues rapidly, and that speed can overwhelm the host cell, which bursts when it is full of the replicated viral particles, which then escape and can infect more cells. Humans do have some drugs to treat some viruses that cause us harm, but they can be a tough target.
Retroviruses use a special enzyme, reverse transcriptase, that makes a retrovirus far worse than just a typical virus. A retrovirus can infect a cell and lie dormant rather than destroying it; the cell goes along replicating its own DNA right along with the genetic information of the virus. It can undergo lots of rounds of replication as well, giving it lots of chances to mutate. Those mutations make it especially hard to fight with drugs. In the video, HIV is explained as one example of a deadly retrovirus. Interestingly, retroviruses have also given humans an evolutionary boost because of their ability to manipulate host DNA.