Mary, I would make the assumption that Chica is negative, and will always be negative and will live a long and happy life. I think that 30% of cats (probably many more) have a natural immunity to FeLV. After two years of living together, it seems that Chica is one of the lucky ones. Don't worry about her as she will pick that up and might feel she has to oblige you. Just celebrate her permanent negative status.
Jane
