In statistics, excess risk is a measure of the relationship between a specified risk factor and a specified outcome (such as contracting a disease).
 It is the difference between two proportions.
 In epidemiology it is typically defined to be the difference between the proportion of subjects in a population with a particular disease who were exposed to a specified risk factor and the proportion of subjects with that same disease who were not exposed.
 That is,
Marek's disease is a highly contagious viral neoplastic disease in chickens.
 It is named after József Marek, a Hungarian veterinarian.
 Marek's disease is caused by an alphaherpesvirus known as 'Marek's disease virus' (MDV) or "Gallid herpesvirus 2" (GaHV-2).
 The disease is characterized by the presence of T cell lymphoma as well as infiltration of nerves and organs by lymphocytes. Viruses "related" to MDV appear to be benign and can be used as vaccine strains to prevent Marek's disease.
 For example, the related Herpesvirus of Turkeys (HVT), causes no apparent disease in turkeys and continues to be used as a vaccine strain for prevention of Marek's disease (see below).
 Birds infected with GaHV-2 can be carriers and shedders of the virus for life.
 Newborn chicks are protected by maternal antibodies for a few weeks.
 After infection, microscopic lesions are present after one to two weeks, and gross lesions are present after three to four weeks.
 The virus is spread in dander from feather follicles and transmitted by inhalation.
Charlotte M. Zeepvat is an author and historian of European royal history.
 She has written five books published by Sutton Publishing, including biographies of Prince Leopold, Duke of Albany and the Romanov Imperial Family.
 An avid collector of original historical photographs, Zeepvat has also produced two collections of royal photographs, one about Queen Victoria and the other about the Romanovs.
Prince Leopold, Duke of Albany, (Leopold George Duncan Albert; 7 April 185328 March 1884) was the eighth child and youngest son of Queen Victoria and Prince Albert of Saxe-Coburg and Gotha.
 Leopold was later created Duke of Albany, Earl of Clarence, and Baron Arklow.
 He had haemophilia, which led to his death at the age of 30.
White band disease is a coral disease that affects acroporid corals and is distinguishable by the white band of dead coral tissue that it forms.
 The disease completely destroys the coral tissue of Caribbean acroporid corals, specifically elkhorn coral ("Acropora palmata") and staghorn coral ("A. cervicornis").
 The disease exhibits a pronounced division between the remaining coral tissue and the exposed coral skeleton.
 These symptoms are similar to white plague, except that white band disease is only found on acroporid corals, and white plague has not been found on any acroporid corals.
 It is part of a class of similar disease known as "white syndromes", many of which may be linked to species of "Vibrio" bacteria.
 While the pathogen for this disease has not been identified, "Vibrio carchariae" may be one of its factors.
 The degradation of coral tissue usually begins at the base of the coral, working its way up to the branch tips, but it can begin in the middle of a branch.
Feline hepatic lipidosis, also known as feline fatty liver syndrome, is one of the most common forms of liver disease of cats.
 The disease officially has no known cause, though obesity is known to increase the risk.
 The disease begins when the cat stops eating from a loss of appetite, forcing the liver to convert body fat into usable energy.
 If this process continues for too long, fat builds up in the cells of the liver, and the disease has officially onset.
 Prognosis varies depending on the stage of the disease, with both a high recovery and mortality rate at different stages.
 The disease is reversible through intense feeding.
 Treatment may involve the insertion of a temporary feeding tube to ensure adequate caloric intake for cats that have stopped eating as a result of this disease.
Disease burden is the impact of a health problem as measured by financial cost, mortality, morbidity, or other indicators.
 It is often quantified in terms of quality-adjusted life years (QALYs) or disability-adjusted life years (DALYs), both of which quantify the number of years lost due to disease (YLDs).
 One DALY can be thought of as one year of healthy life lost, and the overall disease burden can be thought of as a measure of the gap between current health status and the ideal health status (where the individual lives to old age free from disease and disability).
 According to an article published in "The Lancet" in June 2015 low back pain and major depressive disorder were among the top ten causes of YLDs and were the cause of more health loss than diabetes, chronic obstructive pulmonary disease, and asthma combined.
 The study based on data from 188 countries, considered to be the largest and most detailed analysis to quantify levels, patterns, and trends in ill health and disability, concluded that "the proportion of disability-adjusted life years due to YLDs increased globally from 21.1% in 1990 to 31.2% in 2013."
 The environmental burden of disease is defined as the number of DALYs that can be attributed to environmental factors.
 These measures allow for comparison of disease burdens, and have also been used to forecast the possible impacts of health interventions.
 By 2014 DALYs per head were "40% higher in low-income and middle-income regions."
Caroli disease (communicating cavernous ectasia, or congenital cystic dilatation of the intrahepatic biliary tree) is a rare inherited disorder characterized by cystic dilatation (or ectasia) of the bile ducts within the liver.
 There are two patterns of Caroli disease: focal or simple Caroli disease consists of abnormally widened bile ducts affecting an isolated portion of liver.
 The second form is more diffuse, and when associated with portal hypertension and congenital hepatic fibrosis, is often referred to as "Caroli syndrome."
 The underlying differences between the two types are not well understood.
 Caroli disease is also associated with liver failure and polycystic kidney disease.
 The disease affects about one in 1,000,000 people, with more reported cases of Caroli syndrome than of Caroli disease.
The candidate gene approach to conducting genetic association studies focuses on associations between genetic variation within pre-specified genes of interest and phenotypes or disease states.
 This is in contrast to genome-wide association studies (GWAS), which scan the entire genome for common genetic variation.
 Candidate genes are most often selected for study based on "a priori" knowledge of the gene's biological functional impact on the trait or disease in question.
 The rationale behind focusing on allelic variation in specific, biologically relevant regions of the genome is that certain mutations will directly impact the function of the gene in question, and lead to the phenotype or disease state being investigated.
 This approach usually uses the case-control study design to try to answer the question, "Is one allele of a candidate gene more frequently seen in subjects with the disease than in subjects without the disease?"
The natural history of disease is the course a disease takes in individual people from its pathological onset ("inception") until its eventual resolution through complete recovery or death.
 The inception of a disease is not a firmly defined concept.
 The natural history of a disease is sometimes said to start at the moment of exposure to causal agents.
 Knowledge of the natural history of disease ranks alongside causal understanding in importance for disease prevention and control.
 Natural history of disease is one of the major elements of descriptive epidemiology.

Question: Which disease did one of the subjects of Charlotte Zeepvat's writing have?
Answer: haemophilia