Mosquitos clearly love me-- but I have been told I am just more reactive to mosquito bites. Are some people more 'attractive' to mosquitos (get bit more)? Or is it just that they are more reactive? If it is an 'attractive' trait-- is this hereditary?
There is a genetic component to mosquito's attraction to humans.
Female mosquitoes display preferences for certain individuals over others, which is determined by differences in volatile chemicals produced by the human body and detected by mosquitoes. ... Overall, there was a strong narrow-sense heritability of 0.62 (SE 0.124) for relative attraction and 0.67 (0.354) for flight activity based on the average of ten measurements. The results demonstrate an underlying genetic component detectable by mosquitoes through olfaction.
However, analysis of a comparative rating variable "compared with your twin, who is bitten by mosquitoes more often?" indicated a strong genetic influence on frequency of being bitten by mosquitoes, with no significant differences observed between males and females.
I would distinguish two major types of mosquito bites "risk factors": genetic predispositions and transient attractiveness.
Mosquitoes utilize different senses to localise and choose a host. Olfaction seems to be a principal stimuli (constant, gradient measure, simple sensory system). Based form experience from studying other hematophagous ("blood-eating") animals carbon dioxide (CO) is a key feature of breathing living animals. Therefore, CO is an indicator of potential host suitability. Amount of exhaled CO may increase eg. with body mass.
Preferences are made based on excretion of (S)-lactic acid and products of skin bacteria, which may vary between individuals and be attributable to genetic background (eg. the immune system that may affect the composition of skin flora) and transient factors (eg. physical activity).
Skin temperature is another possible stimuli, that mosquitoes can detect. It is correlated with individual's metabolic activity and results from increased blood flow through the skin (increasing chances for tasty meal). So all factors that affect metabolism rate may make individuals more prone for bites, eg. pregnancy, physical activity or fever. The latter is essential for malaria spreading.
Some studies suggest blood type (unlucky type 0) or even beer drinking as an important factors.
As sugar is one of the main blood nutritional resources, it is reasonable to hypothesise that the blood sugar level may correlate with the number of mosquito bites, but relevant literature supporting this is lacking.
Sources and further reading (highly recommended):
Host Preferences of Blood-Feeding Mosquitoes Annual Review of Entomology Vol. 58:433-453 (Volume publication date January 2013)
Landing Preference of Aedes albopictus (Diptera: Culicidae) on Human Skin Among ABO Blood Groups, Secretors or Nonsecretors, and ABH Antigens Journal of Medical Entomology 41(4):796-799. 2004
Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes PLoS ONE 5(3): e9546
Limburger cheese as an attractant for the malaria mosquito Anopheles gambiae s.s. Parasitol Today. 1996 Apr;12(4):159-61.