For most acute viral or bacterial diseases, the major diagnostic aims are either to detect acute infection or to analyse the host's immune status. For many parasitic diseases, diagnostic issues are more complex. This stems from two factors. First, the patient will often have had a lifelong history of contact with the disease. Second, each disease has a wide clinical spectrum ranging from states of latent infection or asymptomatic carriage to acute or chronic pathology, including life-threatening conditions.

Relevant diagnostic questions associated with individual cases might be related to identifying the stage of infection, assessing morbidity, identifying subjects who are at risk of developing severe morbidity, assessing the parasite load in helminth infections, and so on.

For public health purposes, diagnostic issues are different and ask for other diagnostic tools. An epidemiologist has to quantify transmission or assess the impact of control measures on disease incidence, prevalence, overall morbidity or transmission.