Dr Hazel Barton takes Jolyon Jenkins underground, on a hunt for one of the world's most abundant organisms, pelagibacter ubique.
First discovered in the Sargasso Sea in the 1970s, pelagibacter lives on practically nothing.
But can it survive in cave rocks, deep under the Kentucky countryside?
Professor Keith Dumbell has been researching the smallpox virus since 1947, and was part of the successful campaign in the 1960s and '70s to eliminate smallpox from the globe.
In the process he built up the world's largest private collection of smallpox strains.
He tells Jolyon Jenkins about his relationship with the virus, and the sadness he felt when, in the interests of safety, he had to relinquish his collection.
Professor John Oxford is one of the world's top flu experts, and is involved in a race against time to prevent a flu pandemic that could kill millions worldwide.
He thinks the answers may lie in the strain of the virus that broke out after the First World War.
He wants to get samples of tissue from the victims of that outbreak, and is on the hunt for well-preserved bodies.
He tells Jolyon Jenkins why he is so fascinated with a virus he describes as "treacherous".
Professor Dorothy Crawford tells Jolyon Jenkins about her relationship with the Epstein-Barr virus, an elusive bug that causes neck tumours in African children, nasal tumours in Chinese adults, and glandular fever in western teenagers.
Despite researching it for 20 years, she's still puzzled about how a virus can cause such disparate effects.
"I admire it," she says.
"I don't want to give it a personality but it is very clever, amazing.
I also find it rather irritating".
Professor Hugh Pennington at Aberdeen University has studied the bacteria staphylococcus aureus since the 1960s, and was himself infected with it in his nose during an outbreak that swept through British hospitals at that time.
Staph A is now better known as MRSA, the superbug that has evolved resistance to most antibiotics, Professor Pennington tells Jolyon Jenkins about the history of the bug which was discovered by one of his predecessors in Aberdeen in the 19th century, and the series of mistakes that turned a common or garden bug into a major health threat.