Unfortunately, in the US they differentiate between individual insurance plans and group insurance. With group insurance, they have to cover pre-existing conditions if you had a different insurance plan immediately, and if you don't there is a waiting period depending on how long you've been without insurance (waiting period cannot exceed 18 months I believe). The problem is group insurance is only available through groups, i.e. employment, and if you get laid off or fired a) you aren't always guaranteed you can keep the insurance, b) if you can keep the insurance, it is rarely for longer than 18 months and c) it is generally ridiculously expensive -- the last time I did this, to insure a family of 4 with a decent plan it was $1100 per month.
Also unfortunately, the above is the good news with respect to the American insurance system. The bad news is that individual insurance doesn't have to accept anyone -- I knew of one plan that refused to accept someone because they had been treated for hay fever. If they do accept you, they can exclude anything they want, and the exclusion is generally permanent. And the insurance is still expensive.
Finally, the worst news -- we don't have a socialized safety net. Government insurance requires either very low income, very low assets or both, and because the rules differ from state to state, your CI might not even be covered.
