Even as global cases of COVID-19 are on the decline, a number of other viruses such as influenza, respiratory syncytial virus, adenovirus, tuberculosis, and monkeypox are on the rise and have also exhibited strange behaviours in recent months, according to a media report.

Health experts state that reduced exposure due to COVID restrictions may have lowered immunity to infectious diseases, making people more vulnerable to new outbreaks, CNBC reported. The report noted that during COVID the diseases caused by these viruses were subdued.

Flu during the winters of 2020 and 2021 in the US was one of the mildest on record both in terms of deaths and hospitalisations. But as COVID abated and restrictions were removed, the virus-led diseases began ticking upward beginning in February and extending well into summers.

"We've never seen a flu season in the US extend into June," Dr Scott Roberts, associate medical director for infection prevention at Yale-New Haven Hospital, was quoted as saying.

"COVID has clearly had a very big impact on that. Now that people have unmasked, places are opening up, we're seeing viruses behave in very odd ways that they weren't before," he added.

Respiratory syncytial virus, a cold-like virus common during winter months, exhibited an uptick last summer, with cases surging among children in Europe, the US and Japan.

Adenovirus 41, usually responsible for gastrointestinal illness, is suspected to be the reason behind acute hepatitis among young children - infecting about 700 children in 34 countries and claiming 10 lives. Further, Washington state in the US has seen the worst flare-up of tuberculosis in 20 years, the report said.

Another is the monkeypox outbreak which has over 1,000 confirmed and suspected cases emerging in 29 non-endemic countries. At least two genetically distinct variants of the rare viral infection typically found in Central and West Africa have been detected in the US, likely stemming from two different spillover infections from animals to humans,

The World Health Organisation noted earlier last week that the virus, whose symptoms include fever and skin lesions, may have been going undetected in society for "months or possibly a couple of years".

DNA sequencing of the monkeypox virus by researchers at the University of Edinburgh in the UK showed that the virus has been circulating since 2017. The recent outbreak is the first ever to be seen in non-endemic countries outside Africa and is manifest in men who have sex with men, even after not being a sexually transmitted disease. In addition, symptoms are also appearing in new ways.

"Patients are presenting differently than we were previously taught," said Roberts, noting that some infected patients are bypassing initial flu-like symptoms and immediately developing rashes and lesions, specifically and unusually on the genitals and anus.

"There's a lot of unknowns that do make me uneasy. We are seeing very atypical behaviours in a number of ways for a number of viruses," he said. Besides COVID-induced restrictions and mask-wearing over the past two years, the viruses were also missed as public health surveillance that was largely focused on curbing the pandemic, the experts said.

COVID-led lockdowns also curtailed childhood vaccinations, potentially raising the risks of other vaccine-preventable illnesses such as measles and pertussis. "During the COVID pandemic, access to primary care, including childhood vaccinations, was unavailable to many children," Jennifer Horney, professor of epidemiology at the University of Delaware, told CNBC.

"To prevent increases in these diseases, catch-up vaccination campaigns are needed globally," she added.