1. The use of EtxB, CtxB or VtxB free from whole toxin as an immunomodulator for a vaccine against infectious diseases. 2. The use according to claim 1, wherein the immunomodulator is EtxB free from whole toxin. 3. The use according to claim 1, wherein the infectious disease is one for which the infectious agent is a member of the herpes virus family. 4. The use according to claim 3, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected from the group consisting of HSV-1, HSV-2, EBV, VZV, CMV, HHV-6, HHV-7 and HHV-8. 5. The use according to claim 4, wherein the infectious agent is selected from the group consisting of HSV-1, HSV-2, CMV or EBV. 6. The use according to claim 1 or 2, wherein the infectious disease is caused by an infectious agent, and the infectious agent is an influenza virus. 7. The use according to claim 1 or 2, wherein the infectious disease is caused by an infectious agent, and the infectious agent is a parainfluenza virus. 8. The use according to claim 1 or 2, wherein the infectious disease is caused by an infectious agent, and the infectious agent is a respiratory syncytial virus. 9. The use according to claim 1 or 2, wherein the infectious disease is caused by an infectious agent, and the infectious agent is a hepatitis virus. 10. The use according to claim 9, wherein the infectious agent is selected from the group consisting of hepatitis A, B, C and D viruses. 11. The use according to claim 10, wherein the infectious agent is a hepatitis A virus or a hepatitis C virus. 12. The use according to claim 1 or 2, wherein the infectious disease is meningitis. 13. The use according to claim 12, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected form the group consisting of Neisseria meningitidis, Haemophilus influenzae type B and Streptococcus pneumoniae. 14. The use according to claim 1 or 2, wherein the infectious disease is pneumonia or a respiratory tract infection. 15. The use according to claim 14, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected from the group consisting of Streptococcus pneumoniae, Legonella pneumophila and Mycobacterium tuberculosis. 16. The use according to claim 1 or 2, wherein the infectious disease is a sexually-transmitted disease. 17. The use according to claim 16, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected from the group consisting of Neisseria gonnorheae, HIV-1, HIV-2 and Chlamydia trachomatis. 18. The use according to claim 1 or 2, wherein the infectious disease is a gastrointestinal disease. 19. The use according to claim 18, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected from the group consisting of enteropathogenic, enterotoxigenic, enteroinvasive, enterohaemorrhagic and enteroaggregative E. coli, rotavirus, Salmonella enteritidis, Salmonella typhi, Helicobacter pylori, Bacillus cereus, Campylobacter jejuni and Vibrio cholerae. 20. The use according to claim 1 or 2, wherein the infectious disease is a superficial infection. 21. The use according to claim 20, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected from the group consisting of Staphylococcus aureus, Streptococcus pyogenes and Streptococcus mutans. 22. The use according to claim 1 or 2, wherein the infectious disease is a parasitic disease. 23. The use according to claim 22, wherein the infectious disease is caused by an infectious agent, and the infectious agent is selected from the group consisting of malaria, Trypanasoma spp., Toxoplasma gondii, Leishmania donovani and Oncocerca spp. 24. A vaccine composition for use against an infectious disease, which infectious disease is caused by an infectious agent, wherein the vaccine composition comprises an antigenic determinant and an immunomodulator selected from EtxB, CtxB or VtxB free from whole toxin, wherein said antigenic determinant is an antigenic determinant of said infectious agent. 25. A vaccine composition according to claim 24 in which the infectious disease is HSV-1 infection and wherein the antigenic determinant is an antigenic determinant of HSV-1. 26. A vaccine composition according to claim 24 or 25 in which the immunomodulator is EtxB free from whole toxin. 27. A vaccine composition according to claim 24, 25 or 26 in which the immunomodulator and the antigenic determinant are separate moieties. 28. A vaccine composition according to claim 24, 25 or 26 in which the immunomodulator and the antigenic determinant are linked by a bifunctional crosslinking reagent. 29. A kit for vaccination of a mammalian subject against an infectious disease, which kit comprises: a) a vaccine immunomodulator selected from EtxB, CtxB or VtxB free from whole toxin, and b) an antigenic determinant of the infectious disease, for coadministration with the said vaccine immunomodulator. 30. A method of preventing or treating a disease in a host, which method comprises the step of inoculating said host with a vaccine comprising at least one antigenic determinant and an immunomodulator, where the immunomodulator is EtxB, CtxB or VtxB free from whole toxin. 31. The use of EtxB, CtxB or VtxB free from whole toxin to upregulate the production of antibodies at mucosal surfaces. 32. The use of EtxB, CtxB or VtxB free from whole toxin as an immunomodulator in a vaccine, to prolong antigen presentation and give sustained immunological memory in a mammalian subject. 33. A vaccine composition for use against an infectious disease, which infectious disease is caused by an infectious agent, which vaccine comprises an antigenic determinant and a immunomodulator selected from EtxB, CtxB or VtxB free from whole toxin, wherein said antigenic determinant is an antigenic determinant of said infectious agent and wherein the immunomodulator prolongs presentation of the antigenic determinant and gives sustained immunological memory. 34. The use of EtxB, CtxB or VtxB free from whole toxin in a conjugate with antigen or antigenic determinant to target the delivery or said antigen or antigenic determinant to the cytosol or nucleus of an antigen presenting cell. 35. The use of EtxB, CtxB or VtxB free from whole toxin in a conjugate with antigen or antigenic determinant to upregulate the presentation of said antigenic determinant, or an antigenic determinant derived from said antigen, by MHC class I molecules. 36. A vaccine composition which comprises a) EtxB, CtxB, and b) an EBV antigen, for use in the treatment and/or prevention of EBV associated diseases. 37. A therapeutic composition which comprises EtxB, CtxB for use in the treatment of EBV-associated diseases.