Blood test reveals how aging and chronic illness weaken immunity in the elderly

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: For all patients: * Patients aged over 75 hospitalised in one of the geriatric units defined as the research sites; * Patient affiliated to a social security scheme; * Patients or relatives who have been informed and have no objections to the research. To be eligible in the acute situation sub-population: \- Patients presenting with an acute episode on admission or following a stay in the research sites; The acute episode is defined, for example, by an infection, cardiac decompensation, a complicated fall, acute renal failure, any acute or decompensated cardiovascular pathology, any discovery of cancer that has not been stabilised. To be eligible in the stable situation sub-population: \- A patient who has not had an acute episode for at least 7 days. Exclusion Criteria: For all patients: * Patients under legal protection: curatorship, guardianship, safeguard of justice ; * Patients under exclusive comfort care for whom it has been decided not to take a blood sample; * Patients undergoing long-term antibiotic treatment, i.e. with no defined stopping date; * An immunocompromised patient or a patient undergoing immunosuppressive treatment (for example: HIV infection, a patient awaiting solid organ transplantation, a patient undergoing haematopoietic stem cell transplantation, a patient undergoing chemotherapy for a solid tumour or haematological malignancy, a patient suffering from an auto-immune disease and being treated with immunotherapy and/or immunosuppressive therapy and/or biotherapy, an asplenic or hyposplenic patient, a patient undergoing experimental potentially immunosuppressive treatment, a patient undergoing Eculizumab treatment, a patient presenting a hereditary immune deficiency). For patients in an acute clinical situation: \- Patient previously included in the same sub-population.