Question: What is the deadliest bacteria or virus

As Zoe and Kareen have said, rabies is fatal once symptoms show. But rabies is 100% preventable with vaccination (provided this happens before symptoms show). Around 60,000 human rabies deaths are reported worldwide every year (many of these are in children) and the vast majority of humans who die from rabies caught it by being bitten by a rabies infected dog. Thankfully for us, UK is rabies free but as a vet, when I have worked in countries where rabies is endemic, I had regular rabies vaccination before I travelled to protect me against this risk but it’s not financially or logistically possible for governments in these countries to vaccinate every single member of the population. Animal welfare charities and local governments are working together in many countries to vaccinate dogs against rabies and by doing this to avoid it being transmitted to people. Since many dogs are free roaming rather than being kept in a house like we do in the UK you can imagine that this is a huge task (all the dogs have to be individually, humanely caught to give them their vaccination) and because vaccination doesn’t provide lifetime protection (for either dogs or humans) the vaccination programmes have to be repeated every year to maintain the protection and to cover puppies which have been born since the previous vaccination programme.