It certainly isn't a joke; it got me early January (from caving, at a site I'd visited almost 100 times over the preceding 60 month without problem). I'm very pleased that Chris has flagged this up because awareness is everything. The information on the BCA website is very good; taking a print out to the doctor is advisable because not all GPs are especially aware of this bacterium.
Another problem you might find is the "get past the receptionist if you can" issue, which you'll not feel like facing if you've got Leptospirosis. Insist on speaking to the triage doctor and try to have a coherent story; explain that, as a caver, you are in a high risk group. Your aim is to get on the antibiotics as soon as possible. Don't mess about if you think you've got Leptospirosis; if you don't get treated soon enough it can be fatal.
In case it helps - my symptoms were identical to (genuine) influenza without the runny nose for the first 5 days. After that it got really bad (liver inflamed, jaundice, urine looking like Pernod with blackcurrant in, etc). I didn't eat properly for a week and a half and lost a lot of weight. Fortunately my doctor did get me on the right pills just as the really bad symptoms were starting, so I knew I'd be fine eventually. (He actually bypassed the diagnostic blood test, to reduce any delay in starting the antibiotics.) But it's the most ill I've ever felt and I wasn't fully right till mid March.
The good news is that exposure to the Leptospira bacterium may result in the development of immunity to the particular strain which infects you - but not, of course, to any other strains of the bacterium. I was told this by three separate doctors, one of whom is an infectious disease specialist - but it was opinions rather than certainties. However, I've been back to the place where I caught the disease several times since my own infection and I've not become reinfected. I do know of a caver who has had the disease twice though - maybe he was hit by two different strains.
Incidentally a certain person who knows more about illness than most - and who is a regular contributor to this forum - was incredibly helpful to me with advice when I was ill. I got the opportunity to thank him in person last week but I'd also like to express my gratitude here too. He knows who he is!