The number of infectious agents associated with risks of malignant hematologic diseases is non-negligible and include both viruses and bacteria.

The various organisms affect cancer risk either directly by transforming susceptible cells, through chronic antigenic stimulation or by hampering immune function in other ways conducive of cancer development.

Suspicion of an infectious cancer origin may arise because of clustering with other conditions (e.g. immune deficiency), specific environments or settings (e.g. geographic locales) or with exposures (e.g. blood transfusions).

In this context, relatively few studies have addressed clustering of diseases among spouses to generate hypotheses about the relative contributions of environmental and genetic factors to the risk of individual cancer types.

As a prelude to such an exercise aiming specifically at malignant hematologic diseases, we will test an algorithm characterising cohabitation patterns in the Danish population to assess the risk of sexually transmitted diseases in analyses of register data.

Such information will also be relevant to current guidelines for blood donor deferral policies. Specifically, because of the so-called precautionary principle all blood donations are extensively tested for infectious agents and transfusion of blood now carries an extremely low risk of transmission of HIV, hepatitis B and C. The residual risk of HIV transmission in Denmark is estimated to 1:10,000,000 transfusions. However, several deferral criteria have existed for years without studies to prove their relevance.

Aim: To compare the incidence of both known and suspected sexually transmitted diseases between different cohabitation patterns in the Danish population.

Perspectives: The study results can be used to leverage changes in deferral rules in the Danish blood banks to accommodate strong wishes from stakeholders to avoid the perceived discrimination of various minorities. The study can thus have important ethical and political consequences.

Inclusion Criteria:

• All citizens in Denmark

Exclusion Criteria:

• Institutionalized individuals

In Denmark, every citizen is registered with a unique identification number in the Civil Registration System. Adult persons (18 years or older) who resided in Denmark for any period between 1 April 1968 and primo 2018 will be identified and linked to national health registers by using the unique identification number.

Different living arrangements can be defined by using an address-based algorithm to identify each citizen's complete day-by-day cohabitation or marital status history.