Tropical diseases — some of them never before seen in the United States — are marching northward as climate change lets mosquitoes and ticks expand their ranges. But that does not mean that epidemics will break out, scientists say. Whether a few cases explode into a full-fledged outbreak depends on a set of factors far more complex than the weather. The list of scary bug-borne illnesses seems to get longer every year: Lyme, West Nile, Chagas, dengue, chikungunya — and now Zika, the first case of which turned up in Puerto Rico last week. Some factors in the new spread are, for now, unstoppable, scientists say: the weather is hotter; cheap airfares mean humans travel more than they did decades ago; and cities in tropical countries are becoming more crowded, creating nurseries for each disease. But other factors can be manipulated to stop outbreaks: insects can be killed; patients can be cured before they are bitten again; vaccines can be developed; and simple measures like screens, air-conditioning and bug spray can play big roles.