You know what this is. Read the title. This is caused by pathogens invading the joint capsule and producing arthritis. Mainly this is bacteria but can also be caused by fungi or viruses. This can be acute or chronic but coursework has decided to focus on acute bacterial and so we will too.
The disease is characterised by colonisation of the synvoial fluid by bacteria, followed by an influx of inflammatory and immune cells. Effusion into the joint space and erosion of synovial membranes.
Aetiology (acute bacterial septic arthritis)
60-80% of infection is S. aureus but can also be s. epidermidis/streps and enterococci. Neisseria gonorrhoes can cause it following an STI (and this makes up 75% of cases in younger sexually active people in the US). Gram-negatives like pseudomonas can affect the immunocompromised and elderly. Less common is polymicrobialor anaerobic infections and these are mostly form trauma/metastatic infection.