This thesis was based on a collection of my published works. Chapter 1 introduced the idea that infections, both acute and chronic, were risk factors for stroke. Various infections and micro-organisms associated with stroke were discussed. Specifically, the relationship between micro-organisms such as Cytomegalovirus, Helicobacter pylori, dental pathogens, and stroke were discussed. The theories of the pathogenesis of atherosclerosis were presented. Chapter 1 also reviewed the association between Chlamydia pneumoniae, an atypical respiratory pathogen, and atherosclerosis in detail. Chapter 2 introduced the ‘Chlamydia pneumoniae in elderly patients with stroke’ or ‘C-PEPS’ study from which published works in subsequent chapters of this thesis were based. The C-PEPS was a case-control study that investigated the seroprevalence of C pneumoniae in 100 elderly acute stroke and transient ischaemic attack (TIA) patients versus 87 control medical patients. The C-PEPS study showed a high seroprevalence (immunoglobulin IgG) of C pneumoniae infection in both cases and controls. There was no significant association between C pneumoniae seropositivity and stroke / TIA. Chapter 3 presented a study that investigated the reproducibility of a commercial enzyme linked immunosorbent assay (ELISA) kit (SeroCP, Savyon) used in the CPEPS study. The study concluded that SeroCP ELISA had a good reproducibility for the detection of C pneumoniae IgA and moderately good reproducibility for C pneumoniae IgG and IgM. In chapter 4, the ‘Mycoplasma pneumoniae in elderly patients with stroke’ or ‘MPEPS’ case-control study was based on the same cohort of patients as in the C-PEPS study. The M-PEPS study showed a high seroprevalence of M pneumoniae, another atypical respiratory pathogen, in the cohort. However, the study had ruled out M pneumoniae as a major risk factor for stroke / TIA. In chapter 5, the ‘Legionella pneumophila in elderly patients with stroke’ or ‘L-PEPS’ was another case-control study based on the same cohort of patients as in the C-PEPS 3 study. The L-PEPS study established that there was no statistical difference between the seroprevalence of L pneumophila, another atypical respiratory pathogen, in both the stroke / TIA patients and control medical patients. However, when the results of C-PEPS, M-PEPS and L-PEPS studies were analysed together, it appeared that the aggregate number or infectious burden of chronic atypical respiratory infections was associated with the risk of stroke / TIA. Chapter 6 presented a pilot study that investigated the seroprevalence of Coxiella burnetii, another atypical respiratory pathogen, in the same cohort of patients as in the C-PEPS study. Due to very low seropositivity and zero counts, a conclusion could not be made on any association between C burnetii seropositivity and stroke / TIA. Chapter 7 presented the electrocardiographic findings such as rhythms and ischaemic changes of the same cohort of patients as in the C-PEPS study. Atrial fibrillation was the commonest rhythm abnormality in both elderly stroke / TIA cases and medical controls. After adjusment for background history of ischaemic heart disease, there was a statistical trend to suggest an association between ischaemic elctrocardiographic changes and stroke / TIA. Chapter 8 presented a case-control study that investigated the relationsips between socioeconomic deprivation (SED) status, atypical respiratory infections and survival outcome in the same cohort of patients as in the C-PEPS study. The SED status of stroke / TIA cases and controls were similar. Although an association between acute atypical respiratory infection and SED was only found in the control medical patients, acute atypical respiratory infectious burden was found to associate with income deprivation in the whole cohort of elderly stroke / TIA and medical patients. SED on its own had only a modest effect on the association between chronic atypical respiratory infectious burden and stroke / TIA. Elderly patients’ duration of survival after an acute stroke / TIA appeared not to be affected by their background SED status. Chapter 9 was the concluding chapter. The main findings of the published works submitted for the thesis were summarised and discussed. Some directions for future research were also discussed.