The chances of a 5 years boy contacting STD are very little. However, it is not impossible entirely, as STD does not only spread through sexual contact. It spreads through body fluids. A person cannot contact STD by sharing milk with infected person, as STD does not spread by a mere touch. However, risk of infection increases when the noninfected person comes in contact with sores on the body of infected person. Haemophilia is not a STD, but X-linked recessive disease. Sickle cell anaemia is autosomal recessive disorder.
Related Theory
Reproductive health is a state of complete physical, mental and social well-being and not merely the absence of disease relating to the reproductive system and its functions and process, while Reproductive fitness is the individual's reproductive success (equal to average contribution to gene pool).