Probiotics are "live microorganisms which, when administered in adequate amounts, can confer health benefits on the host". Probiotics, which beneficially affect the host by improving the intestinal microbial balance, may affect the immune response, thus boosting the body system to combat against diseases. While numerous microorganisms that may confer health benefits to the host, the most widely studied probiotics are from the Lactobacillus genera. For the past few decades, probiotics have been studied extensively for their beneficial effects mainly on the host's guts. Recently, probiotics are studied for their promising potentials along the gut-brain-axis. The 'gut-brain-axis' is a bidirectional communication system, dynamic for maintaining homeostasis. Many studies show that certain strains of Lactobacillus improved human's health such as reducing the occurrence of diarrhea caused by rotavirus infections, antibiotic treatment, chemotherapy, or lactose intolerance, reduced irritable bowel syndrome symptoms and treating atopic diseases. Lactobacillus have also gained increasing evidences as an effective therapy in preventing respiratory tract infections such as sinusitis, bronchitis and pneumonia. While the precise mechanisms of using Lactobacillus in management of respiratory infections are largely unknown, speculations include: Lactobacillus compete against pathogens; increase the barrier function in respiratory epithelium; immunostimulatory effects by enhancing cellular immunity with increased activity of natural killer cells and macrophages in airways. Lactobacilli, as one of the largest genera for probiotic microorganisms have been researched to alleviate stress, anxiety and/or depression disorders.