Vultures, dogs, can give a laceration or immediately infect.
The logic of the code is as follows:
- if there is no infection yet, then the infection begins to progress from 0% as usual.
- but if there was already an infection, then another 15% is added to what is available.
In your case, there were most likely 2 attacks from a vulture. The first gave infection, the second immediately increased it to 15%.
Also - don't forget that this is just an incubation period - and it's not such a big problem. Except for the fact of infection as it is.