All people are in despair unless they are "true Christians" (and true Christians are very rare). This is not a depressing thought. Rather, the universality of despair indicates that spirituality is a universal quality of human beings.
People may be in despair even if they are not aware that they are in despair. People may be ill but have symptoms only a physician would recognize; similarly, people may be in despair and not know it. Yet whereas a physician could presumably give a patient a clean bill of health, despair may always be lurking beneath appearances. Moreover, whereas physical illnesses can be cured once and for all, despair can always come back to plague a person despite past efforts to overcome it.
Most people go through life concerned only with petty physical and material cares. Yet whether or not one is in despair is the only question of "eternal" significance.
As its title suggests, the main point of Part I.B. ("The Universality of this Sickness") is that despair is a universal condition, whether or not people are aware of it. The only people who are not in despair are those who are aware of despair and combat it with all their energy.
Part I.B. also makes clear that there is a strong link between despair and Christianity. Kierkegaard stresses that despair is the only concern of importance to "eternity." This implies that rooting out despair is the qualification for the eternal life that Christ promised.