The rarest types of infectious disease are probably those that have been, or are very close to, eradication. Smallpox is the only eradicated disease infecting humans, but there are some viruses being held in secure labs for further study. Polio and guinea worm look to be the next to be eradicated, with only 89 and 9 cases respectively last year.
Another set of rare diseases I thought I’d mention transmissible spongiform encephalopathies (TSEs), such as BSE and CJD. These are caused by prions – a sort of badly built protein without any genetic information, that causes proteins in healthy cells to also go ‘wonky’.
Bethany gave a nice answer I would probably say small pox extinct in the wild and would say that prions (pre-ons) have the record for the rarest infectious diseases. The only difficulty regarding prions is that it can take 20-30 years for a symptoms to appear and so it is often difficult to determine how widespread a prion disease has become. This was a serious difficulty with the BSE outbreak we had here in the UK. There is also one prion disease that may be extinct (we say may as they are so difficult to diagnose it’s tough to say for sure) is Kuru which is interesting as the outbreak we know about were limited to a small number of tribes in Papua New Guinea and the outbreak was linked to the consumption of human of human flesh (as if anyone needed a reason not to eat people that’s a pretty good one).
I’ll just mention a few more viruses that have potential as the rarest, in humans at least Hendra virus appears to have only infected 7 humans so far. The problem with viruses and many bacteria is that we often only become aware of them when they infect humans but diseases like Hendra are actually present in other species and in some cases the viruses have evolved with their host for so long that the virus causes no serious symptoms. This makes these viruses even harder to spot as they won’t be known unless an infected animal happens to be one from which a sample is taken. This is part of the reason it is so important for researchers to investigate wild animals to look for evidence of viruses if we know what diseases are common and in what animals, we can make better predictions of potential new disease outbreaks.
So far we have covered rare diseases caused by viruses so I thought if find a bacterial one.
After some searching I came across whipples disease which is caused by Tropheryma whipplei. The disease is so rare that only one in a million develop it and most of them are farmers and is most common in males so for you Lucy it would be a very rare disease. Transmission occurs via person to person and may have some airborne abilities.