The chakra virus (チャクラウイルス Chakurauirusu) is a very dangerous and rare virus that affects a person's ability to mould chakra, turning their own chakra into fever. For this reason, shinobi are very afraid of it. A village that Naruto visited after the invasion of Konoha was infected by this virus, so Naruto was considered infected too. This was later discovered to be incorrect.^[1]

References

1. ↑ Naruto: Shippūden episode 183