In this episode (from series 3) Mark tells his sister that, among other things, Jeremy has chlamydia, to which he replies (almost with pride) '..it's symptom-less'.
However, in series 5 episode 1 Paula tells him she has the disease, that it is symptom-less, and he should consider getting tested. From his surprised reaction to Paula telling him that he might have it (for example, he appears excited when he ponders that maybe he 'could get into this'), it is obvious that he did not know he had it before she tells him now in series 5, and by implication this is the first time he has even considered he might have it.
Also, later in the episode, after Jeremy tells him, Mark says 'You finally caught VD' implying this is the first time that he is aware Jeremy has ever had the disease.
However, in series 5 episode 1 Paula tells him she has the disease, that it is symptom-less, and he should consider getting tested. From his surprised reaction to Paula telling him that he might have it (for example, he appears excited when he ponders that maybe he 'could get into this'), it is obvious that he did not know he had it before she tells him now in series 5, and by implication this is the first time he has even considered he might have it.
Also, later in the episode, after Jeremy tells him, Mark says 'You finally caught VD' implying this is the first time that he is aware Jeremy has ever had the disease.