HTLV‐1 is an oncogenic retrovirus associated with the white cells that cause adult T‐cell leukaemia and is connected with several degenerative neuromuscular syndromes. Screening using the enzyme‐linked immunosorbent assay (ELISA) has been recommended because of concerns relating to the transmission of the virus via blood transfusion and the associated long incubation period of adult T‐cell leukaemia. In 2017, NHSBT changed from universal HTLV screening to selective (for new donors and components that will not be leuco‐depleted) (Bolton‐Maggs [27]). Since the introduction of leucodepletion and screening, transmission of HTLV has been virtually eliminated (Norfolk [202]).

CMV is classified as part of the herpes family and hence has the ability to remain latent within monocytes and dendritic cells and can then reactivate during periods of immunosuppression (Al‐Omari et al. [9]). Approximately 50% of the UK population have antibodies to CMV. Therefore, it is recognized that the virus may be transmitted by transfusion. Although it poses little threat to immunologically competent recipients, in vulnerable patient groups CMV infection can cause significant morbidity and mortality. In March 2012, the Advisory Committee on the Safety of Blood, Tissues and Organs issued a position statement with guidance that the leucodepletion of blood components ‘provides a significant degree of CMV risk reduction’ (SaBTO [240], p.11). This risk reduction is considered adequate protection against transmission of CMV to the majority of patients, including haemopoietic stem cell transplant patients. It is recommend that if an intrauterine, neonate and/or elective transfusion during pregnancy is required that these patients receive CMV‐seronegative components (SaBTO [240]).

Screening for hepatitis B (HBV) is conducted via nucleic acid testing (HBV NAT) (JPAC [121]). SHOT reported one probable and one indeterminate hepatitis B transfusion infection in 2018 (Narayan [172]).

Screening for hepatitis C (HCV) is conducted via nucleic acid testing (HCV NAT) (JPAC [121]). HCV is transmitted primarily via contact with blood or blood products (Friedman [86]). The last reported HCV transfusion‐transmitted infection occurred in 1997 (Bolton‐Maggs [27]).

HIV is a retrovirus that infects and kills helper T‐cells, also known as CD4‐positive lymphocytes. The virus can be transmitted via most blood products, including red cells, platelets, FFP, and factor VIII and IX concentrates. These viruses are not known to be transmitted in albumin, immunoglobulins or antithrombin III products (Barbara and Contreras [13]). The retrovirus invades cells and slowly destroys the immune system, rendering the individual susceptible to opportunistic infections. Since 1983, when it was recognized that the virus could be transmitted via transfusion, actions have been developed to safeguard blood supplies from transmitting the virus that causes acquired immune deficiency syndrome (AIDS). These include the careful screening of donors and the testing of donated blood. The last transmission of HIV‐1 or HIV‐2 due to a blood transfusion occurred in 2002, before the introduction of NAT (nucleic acid testing) screening (Bolton‐Maggs [27]).

Plasmodium falciparum is the most dangerous of the human malarial parasites (Barbara and Contreras [13]). Prevention is maintained by questioning donors about foreign travel, in particular travel to areas where the disease is prevalent (Bishop [22]). The last parasite transfusion‐transmitted infection to be reported to SHOT occurred in 2003 (Bolton‐Maggs [27]).

Known as transmissible spongiform encephalopathies (TSEs), these are a rare group of conditions that cause progressive neurodegeneration in humans and some animal species (Box 12.12). Prion diseases are caused by the presence of an abnormal misfolded aggregated cellular protein (Sigurdson et al. [255]). Key characteristics of vCJD include an accumulation of protease‐resisitant prion protein in lymphoid tissue and the presence of ‘florid’ plaques on neuropathology (Centers for Disease Control and Prevention [40]).

There is evidence to suggest that in TSEs, of which vCJD is one, leucocytes (particularly lymphocytes) are the key cells in transportation of the putative infectious agent to the brain; therefore, as a risk reduction measure, leucodepletion of blood components was introduced in 1999 (Norfolk [202]). There were a total of 178 deaths in cases of definite or probable vCJD in the UK up to 31 December 2017. Analysis of the incidence of vCJD onsets and deaths from January 1994 to December 2011 indicates that a peak has passed (National CJD Research and Surveillance Unit [175]). While this is an encouraging finding, the incidence of vCJD may increase again, particularly if different genetic subgroups with longer incubation periods exist (National CJD Research and Surveillance Unit [175]).

Sepsis can occur as a result of bacteria entering the blood or blood component that is to be infused. The most common bacterial culprit is skin flora (Levy et al. [133]). The blood component that carries the largest risk of causing a bacterial infection is platelets (Bolton‐Maggs [27]). Bacteria can enter at any point from the time of collection, during storage through to administration to the patient.