In The Witch of Blackbird Pond, after John Holbrook leaves to fight a group of Native Americans, what happens to Judith, Kit, and Mercy?
The information you are looking for happens in Chapter 17 of this great historical novel. We are told that it is only five days after John Holbrook's departure that Judith fell ill with fever. This becomes an epidemic in the village as this "mysterious fever" spreads to many others and lots of people are stricken by it. Kit likewise succumbs to the fever, but her experience of it is much shorter and she recovers quickly. However, Mercy falls seriously ill with the fever, worse than either Kit or Judith, and Matthew Wood is forced to go against his better judgement and ask Gershom Bulkeley for help, even though the two of them are at loggerheads about the Crown.
However, as Mercy slowly makes her way back to health, discontent in the village is spreading as some villagers believe that the fever is a result of Hannah Tupper's witchery. They also think she has been helped by Kit, because she recovered so quickly from the fever.