Infected veins, they often present with sustained fevers and persistent bacteremia.
Cannulating the vein, either for medical reason or the use of drugs. As if the two are different.
S. aureus tends to predominate (it ain't called coagulase positive staph for nuthin'; it sure likes to clot) but any organism that can cause a catheter infection can lead to infected clot. Clot of the internal jugular is Lemmiers and often due to F. necrophorum. Clot in the hepatic vein is pylephlebitis, often due to E. coli.
For peripheral thrombophlebitis, the vein needs to be resected. Usually. In the IVDA, 7 days of IV antibiotics followed by po will often work (PubMed).
For septic thrombophlebitis of the great vessels, anticoagulation. Antibiotics depends on what grows in the blood.
Relevant links to my Medscape blog