This isn't true. Viruses are subject to evolutionary pressure even inside a single patient. They don't replicate perfectly (partly because they have to be small and simple, and don't have very good control of the cellular environment they are inside, being invaders and all) and so variants of the particle compete with one another. Because of this, features that might be desired in a bioweapon but are not needed in order for the virus to replicate can get lost.
For instance, a bioweapon virus might contain genes for botulism toxin in order to kill the host. However, copying this gene every generation would diminish the particles ability to replicate, and so variants of the particle that are missing the gene would have a small evolutionary advantage. After just a few patients, the wild version of the virus might have lost this feature.
The FHI's mini advent calendar: counting down through the big five existential risks. The fourth one is an ancient risk, still with us today: pandemics and plagues.
Pandemics
Current understanding: high
Most worrying aspect: the past evidence points to a risky future
The deathrates from infectious diseases follow a power law with a very low exponent. In layman’s terms: there is a reasonable possibility for a plague with an absolutely huge casualty rate. We’ve had close calls in the past: the black death killed around half the population of Europe, while Spanish Influenza infected 27% of all humans and killed one in ten of those, mostly healthy young adults. All the characteristics of an ultimately deadly infection already exist in the wild: anything that combined the deadliness and incubation period of AIDS with the transmissibility of the common cold.
Moreover, we know that we are going to be seeing new diseases and new infections in the future: the only question is how deadly they will be. With modern global travel and transport, these diseases will spread far and wide. Against this, we have better communication and better trans-national institutions and cooperation – but these institutions could easily be overwhelmed, and countries aren’t nearly as well prepared as they need to be.