Can I get critical illness if I already have a health condition?

It depends on what medical conditions you have.

Insurance companies can look at this in different ways. They can make an exclusion for your existing condition covering you for the rest of the defined conditions on that policy that they cover. Insurers can increase the premiums of the policy because of the extra risk to them having to potentially pay out on a successful claim as you may be at a greater risk of contracting the conditions they cover.