The past decade has seen an explosion in the availability and uptake of new technologies enabling the rapid testing, diagnosis and detection of organisms in and beyond the laboratory. At times taking the form of relatively cheap, easily operated and highly transportable devices, these technologies draw on wider developments across the biomedical sciences, including high speed DNA barcoding; PCR (polymerase chain reaction) testing; testing for antibodies and a range of other biomarkers. Such ‘diagnostic devices’ (Street and Kelly, 2020) are being adopted with enthusiasm across a wide range of medical and biological research/practice contexts, including the coronavirus pandemic, where they have enabled the rapid identification of the SARS-CoV-2 virus, alongside high speed research into its origins, evolution and spread. However, as we are seeing in widespread confusion and controversy around ‘coronavirus testing’ in the UK, there are critical gaps between announcing/performing, competently conducting, collecting data from, knowing, understanding and acting upon the various kinds of tests for SARS-CoV-2. This preliminary research aims to investigate the epistemic and regulatory implications of knowing pathogens through new diagnostic devices, comparing the acute case of the COVID-19 pandemic with the effects of diagnostic devices on longstanding – yet neglected – global infectious disease problems: malaria and tuberculosis.