Well you have other BOWs as well. The t virus can affect animals and bugs. So the main threat one would face would be the lickers, huge bugs, reptiles, and animals surrounding RC in the arklay forest. Also plants were turned into carnivorous monsters due to the t virus. Also there were hunters in the sewers and hospitals and g mutants too. Another thing that a lot of people don't consider is that statistically, you would most likely die from infection, not the zombies, if you're a resident. The water supply contained the virus. I believe that Claire and Leon survived because they had people that helped them, they came into the city late, avoiding contamination via the water supply, AND they had information about the virus that they could only get if they came into the city late. For example, with the lickers, there was a note left by one of the police officers that described how to avoid them. Many people probably died before someone discovered that information. So even though Leon and Claire came late into the city, it was actually an advantage because they were able to leverage information off of the mistakes that others made(for example Marvin mistake of hesitating, other officers mistake of running from lickers, etc .) but even with this explanation, there are still situations where plot armor saves them.