The Doobie Brothers are still rockin' down the highway after 51 years, despite four of the band's members being stricken with COVID-19.

"We're back at work, but it's an odd time for anybody doing this," said singer, guitarist and Doobies co-founder Tom Johnston. "The option is to not go out on tour. A lot of people have chosen that option."

With or without the pandemic, the longevity of the Doobies is a happy surprise for Johnston, 73.

"When we started, we didn't even know what was going to happen the next day!" Johnston recalled.

"How does any band know? You're just trying to get it together and move forward. At the start of this band, we hadn't done anything yet and we were playing bars like everyone else. Luckily, we did a demo tape that got us a record deal with Warner Bros. Our first album didn't sell, but the second did. And the rest is history."

Indeed, it is for the Doobies, whose 19th album, "Liberte," was released earlier this month. The band was inducted into the Rock & Roll Hall of Fame last year, albeit virtually because of COVID-19, and is now embarked on its 50th anniversary tour — a year late because of the pandemic-fueled shutdown of live events.

A major commercial force in the 1970s, the Doobies struck gold and platinum with its radio-friendly blend of rock, blues, gospel and soul. The band has sold more than 48 million albums and scored 16 top 40 hits, thanks to such audience favorites as "Jesus Is Just Alright," "Black Water," "Long Train Runnin'," "Listen to the Music" and — after Steely Dan alum Michael McDonald joined up — "What a Fool Believes" and "Minute By Minute."

Some of those songs became prime cover material for bar bands in the 1970s, including a Missouri ensemble that featured future Doobies singer and keyboardist McDonald. He replaced Johnston in the Doobies' lineup in 1975. Johnston returned for the group's 1982 farewell tour and has been a staple of the band since it reunited in 1987.

"I'd been playing Doobies' material for years in nightclubs before I joined the band," McDonald said in a 1987 San Diego Union interview. "They were a major force in music."

Two of the Doobies' co-founders, Johnston and fellow singer-guitarist Patrick Simmons, are still on board today. So is ace guitarist John McFee, who joined in 1979. The band's lineup now also includes veteran Little Feat keyboardist Bill Payne and former Yellowjackets saxophonist Marc Russo, who brings a jazzy elan to some of the group's music.

McDonald rejoined for the 50th anniversary tour, but missed several performances in early September after being diagnosed with COVID-19. He is now back in the fold.

Guitarist McFee and saxophonist Russo also had COVID, said Johnston, who — in early 2020 — was the first member of The Doobies to contract the potentially fatal disease.

"You really, really have to adhere to COVID protocols in order to tour," Johnston said, speaking from Los Angeles last month. "Even if you do adhere, it still happens. If you're really buttoned down, don't leave your hotel room until the gig, wear a mask the whole time until you get on stage, then put the mask back on when you get off stage — and stay in a bubble — it can work.

"It's imperative you do that and don't have any contact outside the bubble. That's the only way you can tour. We did have people in the band get sick with COVID. Without going into a long-winded explanation, I think I know where they got sick; you're not supposed to go into restaurants or bars.

"You can go back and forth about people wearing masks or not, and I won't get into that here. But it really makes it a dicey situation when people don't. As long as you got your masks on and isolate when you're not playing, you should be alright. It's okay to walk around, but not to go hang around with people. And, of course, various sections of the country are more active than others. So, we have to isolate."

But how can the nine touring members of the Doobies isolate on tour buses, which is the mode of transportation the band is using for most ofits 50th anniversary concert trek?

"Well, everybody's wearing a mask, on and off the bus. I know what you're thinking, but that's pretty much the bottom line," Johnston replied.

"Everybody washes their hands regularly. I don't think anybody got sick on a bus. I think it was contact with other musicians (at concerts) or from going into a bar or restaurant. That's all stopped now and I don't think it would happen again that one of us gets sick. I'd be very, very unhappy if it did, because it means we'd have to stop touring."

For its current tour, the Doobies are performing 29 songs over two-and-a-half hours each night. That's a record for the veteran band.

So is the speed with which the Doobies recorded its new album, "Liberte," which was produced and co-written by Bon Jovi's John Shanks. His previous production and songwriting credits range from Alanis Morissette, Van Halen and Stevie Nicks to Bonnie Raitt, Keith Urban and Hilary Duff.

A dozen songs strong, "Liberte" is the first Doobies album that — because of the pandemic — the band made without its members being face to face to write and record the songs.

"It was done really fast, although that's not to say it's the best way to do anything," said Johnston, who co-wrote seven of the new album's songs with Shanks.

If you go

Who: The Doobie Brothers

When: 7:30 p.m. Oct. 28

Where: Mohegan Sun Arena

Tickets: $63.25-$83.25

Visit: mohegansun.com