Not just California. It is a common problem in all major urban areas. More so anywhere - where mass transit is likely to be used, because that's one of the few areas were general population interacts with the homeless population. However, it still isn't likely to be a major source of transmission.
It's not that you may not be exposed to a virus like this - you're exposed to coronaviruses - all the time. Since they are one group of viruses known to cause the common cold, probably everyone here has had a coronavirus. It's highly unlikely that even if you got 2019-nCOV you'd die or even suffer significant effects of it. Because if you're a healthy human being who is properly hydrated, fed, and not suffering from another infection, you're not likely to be taken out of action by such an infection. It's when you're immuno-compromised, nutritionally compromised, etc. that you're more likely to die.
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The thought of the homeless populations being afflicted first and frequently by viral outbreaks is a good one though. It's quite possible that the first populations hit significantly by the AIDS epidemic were not homosexual males like previously thought - but homeless intravenous drug-users in urban areas like New York City.