We probably will never truly know and they are denying anything is even happening.

There’s conflicting reports about what happened, and if it was even plague. One Russian report is that she contracted something in Thailand and then got sick when she was back in Russia. Another is that she accidentally broke a test tube with y. pestis cultures.

If it’s the latter, I’d imagine she hid the accident for fear of punishment until it was too late, and now the Russian government is covering it up.