Infectious diseases are caused by microorganisms like fungi, bacteria, viruses and even parasites. They are contagious and transmitted by insects, animals and by taking contaminated food and water. Chickenpox, measles, typhoid are some of the infectious diseases. Some of the infectious diseases also lead to cancer such as Human papillomavirus causes cervical cancer; lymphoma is caused by infection of Epstein-Barr virus. These infectious diseases are clinically diagnosed by laboratory tests such as blood tests, urine tests, lumbar puncture, throat swabs, computerized tomography, magnetic resonance imaging and biopsies studies. An opportunistic infection is an infection caused by bacterial, viral or fungal pathogens that take advantage of a host with a weak immune system. Mostly these pathogens do not cause disease in a healthy individual that has a normal immune system but in immuno compromised patients. Examples include malnutrition, ageing, leukopenia, HIV, immunosuppressing agents and genetic predisposition. Direct Examination and Techniques: Immunofluorescence, immuno-peroxidase staining, and other immunoassays may detect specific microbial antigens. Genetic probes identify genus- or species-specific DNA or RNA sequences.