Those populations of unvaccinated people give the

virus the chance not only to spread, but to change.那些未接种的人口不仅仅给了病毒一个传播的机会，而且给了一个让病毒变异的机会。

If a virus tries to infect someone with immunity, it may fail, or it may succeed and cause a mild or asymptomatic infection. In that case, it will replicate in response to the pressure from a primed immune system.如果病毒试图感染有免疫力的人，它有可能会失败，也有可能会成功而导致轻度或无症状的感染，在这种情况下，它会在免疫系统的压力下进行复制。

The virus that succeeds will be the virus that makes a random change that makes it look less visible to the immune system.成功的病毒将成为那些随机发生变化的病毒，使其看起来不那么容易被免疫系统发现。

Every time the viruses changes, that gives the virus a different platform to add more mutations. Now we have viruses that spread more efficiently."每次病毒发生变化，就给了病毒一个增加更多突变的不同的平台。现在我们已经出现了一些更具传播力和效率的病毒。

Most of the changes mean nothing to the virus, and some can weaken it. But sometimes, a virus develops a random mutation that gives it an advantage -- better transmissibility, for instance, or more efficient replication, or an ability to infect a great diversity of hosts.大多数变化对病毒来讲是毫无意义的，甚至有的还会削弱它。但是，有些时候病毒会产生一个随机的突变，这个突变会给病毒一种优势--比如更有强的传播力，更有效的复制能力，或者感染多种宿主的能力。

Viruses with an advantage will outcompete other viruses, and will eventually make up the majority of virus particles infecting someone. If that infected person passes the virus to someone else, they'll be passing along the mutant version.具备这些优势的病毒将会在其他病毒中胜出，最终会成为感染某人的主要的病毒粒子，如果这个感染者将病毒传给了其他人，他们也会把变异的病毒传给了别人。

If a mutant version is successful enough, it becomes a variant.如果一个突变足够成功，它就变成了一个变种。

But it has to replicate to do that. An unvaccinated person provides that opportunity.但是它必须要有复制才能做到这一点，而未接种疫苗的人就正好给了病毒一个复制的机会。

"As mutations come up in viruses, the ones that persist are the ones that make it easier for the virus to spread in the population,"当病毒出现突变时，那些持续存在的突变的病毒会更容易在人群中传播。