Germ theory was unknown then and those Spaniards lacked understanding of contagion, well I'm lying, the people then knew about contagion with blood and corpses but not through items like air or blankets. The disease spread, while catastrophic it was fully unintentional. The only accounts like the famed Bartolomé de las Casas described the diseases as "divine punishment" or "mysterious plague", never as a warfare tool.

However with the British, again more than 2 centuries later, there was knowledge and intent as per Jeffrey Amherst and Colonel Henry Bouquet discussing it "Could it not be contrived to send the smallpox among the disaffected tribes?" "I will try to inoculate them with some blankets…" during the Siege of Fort Pitt (1763).

So Spanish are just as bad as the English then. That doesn't help your case.