I can speak to the AIDS side of this with relevant examples, but will do so with a caveat that one of my sources is a bit dated (yet still widely regarded as a solid source). Randy Shilts was a San Francisco journalist who was one of the few newspaper reporters who devoted long-term resources to tracking the AIDS epidemic from its relative infancy in 1981 (note, modern research holds that the virus was being transmitted in Africa as early as 1959, that its roots in the US date back to the late-1960s, and its rapid spread throughout the gay community began in earnest in July 1976). His book, 'And the Band Played On' is a sprawling journalistic investigation that tracks the earliest outbreaks of the disease, along with social, political, and historical components of the epidemic at large.

There's several components to Shilts' book, and they often intermingle, but he examines the work the doctors at the CDC did to track the disease, develop questionnaires, interview those suffering, and run tests to try and develop a theory on just what it was they were looking at. At the same time, Shilts talks about the different volunteer organizations (in San Francisco and New York especially) that sprang up to lobby for political support, donations, and support for those afflicted. There's much more, like a look at the doctors in France as well as in the U.S. that were doing the research to isolate, grow, and later test for the virus (and often fighting for journal prestige against each other), or the doctors in public health or even just private practice that were dealing with the day-to-day realities of seemingly healthy young men coming down with illnesses that wasted them away to nothing before eventual, inevitable death (in those early years, anyway).

It's a very broad but detailed look at how the HIV and AIDS virus spread on a physical level, but also how it drove local, national, and international politics. While it might not be surprising that the budget-slashing Reagan administration didn't devote many resources to the pandemic (Reagan famously did not even say the word "AIDS" in public until roughly 6 years into the epidemic), it might surprise many that there was a very vocal portion of the LGBTQ+ community that fought against what might be considered common-sense actions to curb the spread of the disease. Although doctors knew as early as 1982 that sexual transmission was the likely cause of the illness, there was a sustained, lengthy battle to keep bathhouses in both New York and San Francisco open. And it wasn't just the bathhouse owners that fought this fight, but the patrons as well.

For doctors, this was a horrific scenario, because again, although it couldn't be conclusively proven in those early days without a test, the infection clusters pointed to the bathhouses as a driving factor in the skyrocketing infection rates. Shilts devotes a great deal of time to the struggle that doctors faced in trying to close these bathhouses down, fighting against the gay community (who thought the move was a form of repression and a path towards institutionalized discrimination, which you can't really blame them for believing), but also the public health departments that didn't want to take the political heat from a voting block with a great deal of power within the districts/cities.

Although literature and signs warning against unprotected sex was widely distributed as early as 1983 (and a few bathhouses were closed in 1984), and although business did begin to suffer in the bathhouses after 1985, in San Francisco, they wouldn't be closed until 1987.

There's a lot of reasons this happened. Part of the reason was that it didn't seem real for a lot of men for those first few years, owing to the long incubation period for some victims of the virus (so in 1983, some of the men who had been infected in 1980 weren't showing symptoms, and might not for a few more years). Shilts talks about the attitude of the gay community in those early years, and how the handful of men who contracted the disease didn't seem like enough of a deterrent to the scores of others who appeared healthy.

Shilts talks about denial a lot, too, for even when it was known that sexual transmission or blood contact was the likely culprit, without a test, people believed what they wanted to believe. This idea that it's not real unless it is conclusively proven beyond a doubt played into this not just on a personal level, but from the business side of things, too. The blood industry famously refused to test for Hepatitis antibodies to screen for possible AIDS contamination even though that Hepatitis test proved reliable in something like 80% of cases. They knew as early as 1982 that the blood they were distributing to hemophiliacs was likely tainted (the CDC did amazing work tracking down victims who could have contracted AIDS no other way), but did not begin any kind of testing until 1985. It was a purely financial decision, and it cost thousands their lives.

So, to answer OP's question, yes: while there weren't "deniers" per se, there were plenty of people on all sides of the social and political spectrum who were presented with very real evidence and options to slow the spread of the disease, and for any number of reasons, did little or nothing to curb it. While there was indeed some confusion in the earliest days what might be spreading the illness (a theory went around in the earliest days that it was due to bad poppers), as early as late-1981 and certainly by 1982, there was enough information to start putting preventative measures in place that might have saved millions of lives. Many gay men ignored the advice for a multitude of reasons (chief among them that the reality was too horrible to confront, which again, is entirely understandable). Public health officials were hesitant to close the bathhouses for fear of political backlash. Scientists didn't want to risk their reputations in a lot of cases by claiming something that couldn't be conclusively proven (though some doctors put those reputations on the line to speak out, chief among them Selma Dritz, Don Francis, William Darrow, among others).