Almost all existing vaccines work against bacteria and viruses, there are currently no vaccines against fungi anywhere in the world, and there is just one type of vaccine – the anti-malarial one – against parasites. Parasites often have sophisticated ways of evading a host's immune system. They adapt to their hosts during complex life cycles, involving several developmental stages. "Each one of those stages, either in an animal or a human host, has different proteins that are expressed in the different life stages. And that makes it more complicated to identify what the targets could be of the parasite for development of a vaccine," says O'Brien.

This life cycle complexity is one of the reasons that parasites aren't always easy to culture in a laboratory. In other words, it can be tough to simply produce more of them under controlled conditions, at the right stage of development, for research purposes. Possibilities including cultivating parasites in research animals or in a laboratory, rather than in their usual hosts.

They are also much more genetically diverse, and usually affect lower income countries. But various vaccines for different parasites hold promise. And some of the vaccines' technology might work on eventual HIV vaccines, too.