If a dog or cat bites a human, the animal must be observed for ten days to see if symptoms of rabies develop. This is necessary even if it has been vaccinated, as very rarely, vaccination fails to protect an animal and it develops rabies disease. At the longest, a dog or cat can have rabies virus in its saliva for only 2-3 days before it develops rabies symptoms. Therefore we know that if a dog or cat remains healthy for at least 10 days after it bites someone, it could not have had the rabies virus in its saliva at the time of the bite. If an animal does not have virus in its saliva, it cannot transmit the disease through a bite.